diff --git a/lib/6to5/browser.js b/lib/6to5/browser.js
index ccf6d54ed4..18a7fd0f28 100644
--- a/lib/6to5/browser.js
+++ b/lib/6to5/browser.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var transform = module.exports = require("./transformation/transform");
 
 transform.version = require("../../package").version;
diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 926cf0e427..f9a6eb7d5e 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = File;
 
 var SHEBANG_REGEX = /^\#\!.*/;
diff --git a/lib/6to5/generation/buffer.js b/lib/6to5/generation/buffer.js
index 2fadf5db33..26e6bcc9aa 100644
--- a/lib/6to5/generation/buffer.js
+++ b/lib/6to5/generation/buffer.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Buffer;
 
 var util = require("../util");
diff --git a/lib/6to5/generation/generator.js b/lib/6to5/generation/generator.js
index 39f92e1491..778a590e74 100644
--- a/lib/6to5/generation/generator.js
+++ b/lib/6to5/generation/generator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = function (ast, opts, code) {
   var gen = new CodeGenerator(ast, opts, code);
   return gen.generate();
diff --git a/lib/6to5/generation/generators/base.js b/lib/6to5/generation/generators/base.js
index 5d7eb6f412..a9e8bd51af 100644
--- a/lib/6to5/generation/generators/base.js
+++ b/lib/6to5/generation/generators/base.js
@@ -1,3 +1,5 @@
+"use strict";
+
 exports.File = function (node, print) {
   print(node.program);
 };
diff --git a/lib/6to5/generation/generators/classes.js b/lib/6to5/generation/generators/classes.js
index f7df307f62..df4ecd557a 100644
--- a/lib/6to5/generation/generators/classes.js
+++ b/lib/6to5/generation/generators/classes.js
@@ -1,3 +1,5 @@
+"use strict";
+
 exports.ClassExpression =
 exports.ClassDeclaration = function (node, print) {
   this.push("class");
diff --git a/lib/6to5/generation/generators/comprehensions.js b/lib/6to5/generation/generators/comprehensions.js
index 1fe81bbbe4..feeef3c2de 100644
--- a/lib/6to5/generation/generators/comprehensions.js
+++ b/lib/6to5/generation/generators/comprehensions.js
@@ -1,3 +1,5 @@
+"use strict";
+
 exports.ComprehensionBlock = function (node, print) {
   this.keyword("for");
   this.push("(");
diff --git a/lib/6to5/generation/generators/expressions.js b/lib/6to5/generation/generators/expressions.js
index ebe3fd9b7c..5989e8f8d0 100644
--- a/lib/6to5/generation/generators/expressions.js
+++ b/lib/6to5/generation/generators/expressions.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var util = require("../../util");
 var t    = require("../../types");
 var _    = require("lodash");
diff --git a/lib/6to5/generation/generators/flow.js b/lib/6to5/generation/generators/flow.js
index 0e5c3c0cc2..cc3d481d1a 100644
--- a/lib/6to5/generation/generators/flow.js
+++ b/lib/6to5/generation/generators/flow.js
@@ -1,3 +1,5 @@
+"use strict";
+
 exports.AnyTypeAnnotation =
 exports.ArrayTypeAnnotation =
 exports.BooleanTypeAnnotation =
diff --git a/lib/6to5/generation/generators/jsx.js b/lib/6to5/generation/generators/jsx.js
index 5460660515..d372cbc74b 100644
--- a/lib/6to5/generation/generators/jsx.js
+++ b/lib/6to5/generation/generators/jsx.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/generation/generators/methods.js b/lib/6to5/generation/generators/methods.js
index e147b20a06..40a4e52b89 100644
--- a/lib/6to5/generation/generators/methods.js
+++ b/lib/6to5/generation/generators/methods.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports._params = function (node, print) {
diff --git a/lib/6to5/generation/generators/modules.js b/lib/6to5/generation/generators/modules.js
index 16f6a01d83..6e8a587efb 100644
--- a/lib/6to5/generation/generators/modules.js
+++ b/lib/6to5/generation/generators/modules.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/generation/generators/playground.js b/lib/6to5/generation/generators/playground.js
index b97e2d6184..d11aa04ee6 100644
--- a/lib/6to5/generation/generators/playground.js
+++ b/lib/6to5/generation/generators/playground.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var _ = require("lodash");
 
 _.each(["BindMemberExpression", "BindFunctionExpression"], function (type) {
diff --git a/lib/6to5/generation/generators/statements.js b/lib/6to5/generation/generators/statements.js
index 715858e870..6cc66dd255 100644
--- a/lib/6to5/generation/generators/statements.js
+++ b/lib/6to5/generation/generators/statements.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var util = require("../../util");
 var t    = require("../../types");
 
diff --git a/lib/6to5/generation/generators/template-literals.js b/lib/6to5/generation/generators/template-literals.js
index b723a2b600..4bda97505f 100644
--- a/lib/6to5/generation/generators/template-literals.js
+++ b/lib/6to5/generation/generators/template-literals.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var _ = require("lodash");
 
 exports.TaggedTemplateExpression = function (node, print) {
diff --git a/lib/6to5/generation/generators/types.js b/lib/6to5/generation/generators/types.js
index 4a58375a8d..405fe31720 100644
--- a/lib/6to5/generation/generators/types.js
+++ b/lib/6to5/generation/generators/types.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var _ = require("lodash");
 
 exports.Identifier = function (node) {
diff --git a/lib/6to5/generation/node/index.js b/lib/6to5/generation/node/index.js
index 179b3015d7..ba6cc61f78 100644
--- a/lib/6to5/generation/node/index.js
+++ b/lib/6to5/generation/node/index.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Node;
 
 var whitespace = require("./whitespace");
diff --git a/lib/6to5/generation/node/parentheses.js b/lib/6to5/generation/node/parentheses.js
index 5518086e27..391f8ffc5c 100644
--- a/lib/6to5/generation/node/parentheses.js
+++ b/lib/6to5/generation/node/parentheses.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/generation/node/whitespace.js b/lib/6to5/generation/node/whitespace.js
index d1defd1cfe..8deca5a4b2 100644
--- a/lib/6to5/generation/node/whitespace.js
+++ b/lib/6to5/generation/node/whitespace.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var _ = require("lodash");
 var t = require("../../types");
 
diff --git a/lib/6to5/generation/position.js b/lib/6to5/generation/position.js
index cae67ca356..df85e0a332 100644
--- a/lib/6to5/generation/position.js
+++ b/lib/6to5/generation/position.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Position;
 
 function Position() {
diff --git a/lib/6to5/generation/source-map.js b/lib/6to5/generation/source-map.js
index 34f5d9d0cd..2dc922f024 100644
--- a/lib/6to5/generation/source-map.js
+++ b/lib/6to5/generation/source-map.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = SourceMap;
 
 var sourceMap = require("source-map");
diff --git a/lib/6to5/generation/whitespace.js b/lib/6to5/generation/whitespace.js
index aa0993bc2a..30855472d4 100644
--- a/lib/6to5/generation/whitespace.js
+++ b/lib/6to5/generation/whitespace.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Whitespace;
 
 var _    = require("lodash");
@@ -106,4 +108,4 @@ Whitespace.prototype.getNewlinesBetween = function (startToken, endToken) {
   }
 
   return lines;
-};
\ No newline at end of file
+};
diff --git a/lib/6to5/index.js b/lib/6to5/index.js
index 7e3bac2922..79a33a1f0b 100644
--- a/lib/6to5/index.js
+++ b/lib/6to5/index.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var transform = require("./transformation/transform");
 var util      = require("./util");
 var fs        = require("fs");
diff --git a/lib/6to5/patch.js b/lib/6to5/patch.js
index 76e65bd38f..ac1ff2670b 100644
--- a/lib/6to5/patch.js
+++ b/lib/6to5/patch.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("./types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/polyfill.js b/lib/6to5/polyfill.js
index c8d5124f2b..7a893bd69c 100644
--- a/lib/6to5/polyfill.js
+++ b/lib/6to5/polyfill.js
@@ -1,2 +1,4 @@
+"use strict";
+
 require("core-js/shim");
 require("regenerator/runtime");
diff --git a/lib/6to5/register-browser.js b/lib/6to5/register-browser.js
index 7f7ab7a552..bd18a5fbc9 100644
--- a/lib/6to5/register-browser.js
+++ b/lib/6to5/register-browser.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // required to safely use 6to5/register within a browserify codebase
 
 module.exports = function () {};
diff --git a/lib/6to5/register.js b/lib/6to5/register.js
index 8b4d99d915..6d1b317bd8 100644
--- a/lib/6to5/register.js
+++ b/lib/6to5/register.js
@@ -1,3 +1,5 @@
+"use strict";
+
 require("./polyfill");
 
 var sourceMapSupport = require("source-map-support");
diff --git a/lib/6to5/runtime-generator.js b/lib/6to5/runtime-generator.js
index 8ea86c5c99..fa44638215 100644
--- a/lib/6to5/runtime-generator.js
+++ b/lib/6to5/runtime-generator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var generator = require("./generation/generator");
 var util      = require("./util");
 var File      = require("./file");
diff --git a/lib/6to5/to-fast-properties.js b/lib/6to5/to-fast-properties.js
index a7499fdff5..c82e9cbe4b 100644
--- a/lib/6to5/to-fast-properties.js
+++ b/lib/6to5/to-fast-properties.js
@@ -1,3 +1,5 @@
+"use strict";
+
 /**
  * A trick from Bluebird to force V8 to use fast properties for an object.
  * Read more: http://stackoverflow.com/questions/24987896/
diff --git a/lib/6to5/transformation/helpers/build-binary-assignment-operator-transformer.js b/lib/6to5/transformation/helpers/build-binary-assignment-operator-transformer.js
index 96743c8bdc..5822026ec6 100644
--- a/lib/6to5/transformation/helpers/build-binary-assignment-operator-transformer.js
+++ b/lib/6to5/transformation/helpers/build-binary-assignment-operator-transformer.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var explode = require("./explode-assignable-expression");
 var t       = require("../../types");
 
diff --git a/lib/6to5/transformation/helpers/build-comprehension.js b/lib/6to5/transformation/helpers/build-comprehension.js
index 9ba979caef..d390b7afde 100644
--- a/lib/6to5/transformation/helpers/build-comprehension.js
+++ b/lib/6to5/transformation/helpers/build-comprehension.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 module.exports = function build(node, buildBody) {
diff --git a/lib/6to5/transformation/helpers/build-conditional-assignment-operator-transformer.js b/lib/6to5/transformation/helpers/build-conditional-assignment-operator-transformer.js
index adb097052f..9137b31b23 100644
--- a/lib/6to5/transformation/helpers/build-conditional-assignment-operator-transformer.js
+++ b/lib/6to5/transformation/helpers/build-conditional-assignment-operator-transformer.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var explode = require("./explode-assignable-expression");
 var t       = require("../../types");
 
diff --git a/lib/6to5/transformation/helpers/explode-assignable-expression.js b/lib/6to5/transformation/helpers/explode-assignable-expression.js
index eecc696b78..9412f85390 100644
--- a/lib/6to5/transformation/helpers/explode-assignable-expression.js
+++ b/lib/6to5/transformation/helpers/explode-assignable-expression.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 var getObjRef = function (node, nodes, file, scope) {
diff --git a/lib/6to5/transformation/helpers/name-method.js b/lib/6to5/transformation/helpers/name-method.js
index 14db11b3e9..34ea75fbf1 100644
--- a/lib/6to5/transformation/helpers/name-method.js
+++ b/lib/6to5/transformation/helpers/name-method.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var util     = require("../../util");
 var t        = require("../../types");
diff --git a/lib/6to5/transformation/helpers/remap-async-to-generator.js b/lib/6to5/transformation/helpers/remap-async-to-generator.js
index 7868e86ec7..7426862f71 100644
--- a/lib/6to5/transformation/helpers/remap-async-to-generator.js
+++ b/lib/6to5/transformation/helpers/remap-async-to-generator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var t        = require("../../types");
 
diff --git a/lib/6to5/transformation/helpers/replace-supers.js b/lib/6to5/transformation/helpers/replace-supers.js
index 4ca7009be4..4ec70cf6b5 100644
--- a/lib/6to5/transformation/helpers/replace-supers.js
+++ b/lib/6to5/transformation/helpers/replace-supers.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = ReplaceSupers;
 
 var traverse = require("../../traverse");
diff --git a/lib/6to5/transformation/helpers/use-strict.js b/lib/6to5/transformation/helpers/use-strict.js
index 6ef8e3baf0..7709528a41 100644
--- a/lib/6to5/transformation/helpers/use-strict.js
+++ b/lib/6to5/transformation/helpers/use-strict.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.has = function (node) {
diff --git a/lib/6to5/transformation/modules/_default.js b/lib/6to5/transformation/modules/_default.js
index 1485d27f03..989d7b3f94 100644
--- a/lib/6to5/transformation/modules/_default.js
+++ b/lib/6to5/transformation/modules/_default.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = DefaultFormatter;
 
 var traverse = require("../../traverse");
diff --git a/lib/6to5/transformation/modules/_strict.js b/lib/6to5/transformation/modules/_strict.js
index 2531d64422..160f30b547 100644
--- a/lib/6to5/transformation/modules/_strict.js
+++ b/lib/6to5/transformation/modules/_strict.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var util = require("../../util");
 
 module.exports = function (Parent) {
diff --git a/lib/6to5/transformation/modules/amd-strict.js b/lib/6to5/transformation/modules/amd-strict.js
index 725d67325d..a055b94763 100644
--- a/lib/6to5/transformation/modules/amd-strict.js
+++ b/lib/6to5/transformation/modules/amd-strict.js
@@ -1 +1,3 @@
+"use strict";
+
 module.exports = require("./_strict")(require("./amd"));
diff --git a/lib/6to5/transformation/modules/amd.js b/lib/6to5/transformation/modules/amd.js
index f0aedd7e67..3940c9d18d 100644
--- a/lib/6to5/transformation/modules/amd.js
+++ b/lib/6to5/transformation/modules/amd.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = AMDFormatter;
 
 var DefaultFormatter = require("./_default");
diff --git a/lib/6to5/transformation/modules/common-strict.js b/lib/6to5/transformation/modules/common-strict.js
index 54608375de..bb7b5c944c 100644
--- a/lib/6to5/transformation/modules/common-strict.js
+++ b/lib/6to5/transformation/modules/common-strict.js
@@ -1 +1,3 @@
+"use strict";
+
 module.exports = require("./_strict")(require("./common"));
diff --git a/lib/6to5/transformation/modules/common.js b/lib/6to5/transformation/modules/common.js
index f62cb03883..8e6044f2e9 100644
--- a/lib/6to5/transformation/modules/common.js
+++ b/lib/6to5/transformation/modules/common.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = CommonJSFormatter;
 
 var DefaultFormatter = require("./_default");
diff --git a/lib/6to5/transformation/modules/ignore.js b/lib/6to5/transformation/modules/ignore.js
index 3753440eed..64c902afe7 100644
--- a/lib/6to5/transformation/modules/ignore.js
+++ b/lib/6to5/transformation/modules/ignore.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = IgnoreFormatter;
 
 var t = require("../../types");
diff --git a/lib/6to5/transformation/modules/system.js b/lib/6to5/transformation/modules/system.js
index d0650a69c2..85f64662a9 100644
--- a/lib/6to5/transformation/modules/system.js
+++ b/lib/6to5/transformation/modules/system.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = SystemFormatter;
 
 var AMDFormatter = require("./amd");
diff --git a/lib/6to5/transformation/modules/umd-strict.js b/lib/6to5/transformation/modules/umd-strict.js
index c14d6e4076..6104513e81 100644
--- a/lib/6to5/transformation/modules/umd-strict.js
+++ b/lib/6to5/transformation/modules/umd-strict.js
@@ -1 +1,3 @@
+"use strict";
+
 module.exports = require("./_strict")(require("./umd"));
diff --git a/lib/6to5/transformation/modules/umd.js b/lib/6to5/transformation/modules/umd.js
index d0122f4f68..f5c068bb64 100644
--- a/lib/6to5/transformation/modules/umd.js
+++ b/lib/6to5/transformation/modules/umd.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = UMDFormatter;
 
 var AMDFormatter = require("./amd");
diff --git a/lib/6to5/transformation/transform.js b/lib/6to5/transformation/transform.js
index 838ee5a90d..2a5cbb26a2 100644
--- a/lib/6to5/transformation/transform.js
+++ b/lib/6to5/transformation/transform.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = transform;
 
 var Transformer = require("./transformer");
diff --git a/lib/6to5/transformation/transformer.js b/lib/6to5/transformation/transformer.js
index dbcd0f9d10..d0a4e0dd0a 100644
--- a/lib/6to5/transformation/transformer.js
+++ b/lib/6to5/transformation/transformer.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Transformer;
 
 var traverse = require("../traverse");
diff --git a/lib/6to5/transformation/transformers/_alias-functions.js b/lib/6to5/transformation/transformers/_alias-functions.js
index 1d2bf585f6..836bf8f362 100644
--- a/lib/6to5/transformation/transformers/_alias-functions.js
+++ b/lib/6to5/transformation/transformers/_alias-functions.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var t        = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/_block-hoist.js b/lib/6to5/transformation/transformers/_block-hoist.js
index cc17209797..6e1ab91033 100644
--- a/lib/6to5/transformation/transformers/_block-hoist.js
+++ b/lib/6to5/transformation/transformers/_block-hoist.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var useStrict = require("../helpers/use-strict");
 var _         = require("lodash");
 
diff --git a/lib/6to5/transformation/transformers/_declarations.js b/lib/6to5/transformation/transformers/_declarations.js
index 61852930b6..07974b32a2 100644
--- a/lib/6to5/transformation/transformers/_declarations.js
+++ b/lib/6to5/transformation/transformers/_declarations.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var useStrict = require("../helpers/use-strict");
 var t         = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/_module-formatter.js b/lib/6to5/transformation/transformers/_module-formatter.js
index fb7c7d4a90..503543171e 100644
--- a/lib/6to5/transformation/transformers/_module-formatter.js
+++ b/lib/6to5/transformation/transformers/_module-formatter.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var transform = require("../transform");
 
 exports.ast = {
diff --git a/lib/6to5/transformation/transformers/es6-arrow-functions.js b/lib/6to5/transformation/transformers/es6-arrow-functions.js
index 40f9ab9906..e822b42707 100644
--- a/lib/6to5/transformation/transformers/es6-arrow-functions.js
+++ b/lib/6to5/transformation/transformers/es6-arrow-functions.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.ArrowFunctionExpression = function (node) {
diff --git a/lib/6to5/transformation/transformers/es6-classes.js b/lib/6to5/transformation/transformers/es6-classes.js
index 432b0ff2ea..e320d4634e 100644
--- a/lib/6to5/transformation/transformers/es6-classes.js
+++ b/lib/6to5/transformation/transformers/es6-classes.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var ReplaceSupers = require("../helpers/replace-supers");
 var nameMethod    = require("../helpers/name-method");
 var util          = require("../../util");
diff --git a/lib/6to5/transformation/transformers/es6-computed-property-names.js b/lib/6to5/transformation/transformers/es6-computed-property-names.js
index 3789ca1d23..9d08092830 100644
--- a/lib/6to5/transformation/transformers/es6-computed-property-names.js
+++ b/lib/6to5/transformation/transformers/es6-computed-property-names.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.ObjectExpression = function (node, parent, scope, context, file) {
diff --git a/lib/6to5/transformation/transformers/es6-constants.js b/lib/6to5/transformation/transformers/es6-constants.js
index 096eee9097..7be895f0f2 100644
--- a/lib/6to5/transformation/transformers/es6-constants.js
+++ b/lib/6to5/transformation/transformers/es6-constants.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var t        = require("../../types");
 var _        = require("lodash");
diff --git a/lib/6to5/transformation/transformers/es6-default-parameters.js b/lib/6to5/transformation/transformers/es6-default-parameters.js
index 4b03d56295..408c1a7e63 100644
--- a/lib/6to5/transformation/transformers/es6-default-parameters.js
+++ b/lib/6to5/transformation/transformers/es6-default-parameters.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var util     = require("../../util");
 var t        = require("../../types");
diff --git a/lib/6to5/transformation/transformers/es6-destructuring.js b/lib/6to5/transformation/transformers/es6-destructuring.js
index 910eed82a8..3a3e27c4b5 100644
--- a/lib/6to5/transformation/transformers/es6-destructuring.js
+++ b/lib/6to5/transformation/transformers/es6-destructuring.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // TODO: Clean up
 
 var t = require("../../types");
diff --git a/lib/6to5/transformation/transformers/es6-for-of.js b/lib/6to5/transformation/transformers/es6-for-of.js
index 9018e995af..3eed21d15d 100644
--- a/lib/6to5/transformation/transformers/es6-for-of.js
+++ b/lib/6to5/transformation/transformers/es6-for-of.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var util = require("../../util");
 var t    = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/es6-generators.js b/lib/6to5/transformation/transformers/es6-generators.js
index caeeec5ef6..6d8124b709 100644
--- a/lib/6to5/transformation/transformers/es6-generators.js
+++ b/lib/6to5/transformation/transformers/es6-generators.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var regenerator = require("regenerator");
 
 exports.ast = {
diff --git a/lib/6to5/transformation/transformers/es6-let-scoping.js b/lib/6to5/transformation/transformers/es6-let-scoping.js
index 8533f9d1d5..14849d6c7c 100644
--- a/lib/6to5/transformation/transformers/es6-let-scoping.js
+++ b/lib/6to5/transformation/transformers/es6-let-scoping.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var util     = require("../../util");
 var t        = require("../../types");
diff --git a/lib/6to5/transformation/transformers/es6-modules.js b/lib/6to5/transformation/transformers/es6-modules.js
index a9b60473f2..b59b191323 100644
--- a/lib/6to5/transformation/transformers/es6-modules.js
+++ b/lib/6to5/transformation/transformers/es6-modules.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.ast = {
diff --git a/lib/6to5/transformation/transformers/es6-property-method-assignment.js b/lib/6to5/transformation/transformers/es6-property-method-assignment.js
index 776b6172cc..4a740e2f83 100644
--- a/lib/6to5/transformation/transformers/es6-property-method-assignment.js
+++ b/lib/6to5/transformation/transformers/es6-property-method-assignment.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var nameMethod = require("../helpers/name-method");
 var util       = require("../../util");
 var t          = require("../../types");
diff --git a/lib/6to5/transformation/transformers/es6-property-name-shorthand.js b/lib/6to5/transformation/transformers/es6-property-name-shorthand.js
index 8614d69c84..006fc9325b 100644
--- a/lib/6to5/transformation/transformers/es6-property-name-shorthand.js
+++ b/lib/6to5/transformation/transformers/es6-property-name-shorthand.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/transformation/transformers/es6-rest-parameters.js b/lib/6to5/transformation/transformers/es6-rest-parameters.js
index c618394c51..89bafc063a 100644
--- a/lib/6to5/transformation/transformers/es6-rest-parameters.js
+++ b/lib/6to5/transformation/transformers/es6-rest-parameters.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var util = require("../../util");
 var t    = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/es6-spread.js b/lib/6to5/transformation/transformers/es6-spread.js
index 4fa64b5e2b..fff975918c 100644
--- a/lib/6to5/transformation/transformers/es6-spread.js
+++ b/lib/6to5/transformation/transformers/es6-spread.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/transformation/transformers/es6-template-literals.js b/lib/6to5/transformation/transformers/es6-template-literals.js
index d2f360b104..682fbbe85b 100644
--- a/lib/6to5/transformation/transformers/es6-template-literals.js
+++ b/lib/6to5/transformation/transformers/es6-template-literals.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 var buildBinaryExpression = function (left, right) {
diff --git a/lib/6to5/transformation/transformers/es6-unicode-regex.js b/lib/6to5/transformation/transformers/es6-unicode-regex.js
index d85bdf4e3b..1c1cf36089 100644
--- a/lib/6to5/transformation/transformers/es6-unicode-regex.js
+++ b/lib/6to5/transformation/transformers/es6-unicode-regex.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var rewritePattern = require("regexpu/rewrite-pattern");
 var _              = require("lodash");
 
diff --git a/lib/6to5/transformation/transformers/es7-abstract-references.js b/lib/6to5/transformation/transformers/es7-abstract-references.js
index 39003b914c..f6c7eefb7e 100644
--- a/lib/6to5/transformation/transformers/es7-abstract-references.js
+++ b/lib/6to5/transformation/transformers/es7-abstract-references.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // https://github.com/zenparsing/es-abstract-refs
 
 var util = require("../../util");
diff --git a/lib/6to5/transformation/transformers/es7-array-comprehension.js b/lib/6to5/transformation/transformers/es7-array-comprehension.js
index e50b135d0b..46b537f5f9 100644
--- a/lib/6to5/transformation/transformers/es7-array-comprehension.js
+++ b/lib/6to5/transformation/transformers/es7-array-comprehension.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var buildComprehension = require("../helpers/build-comprehension");
 var traverse           = require("../../traverse");
 var util               = require("../../util");
diff --git a/lib/6to5/transformation/transformers/es7-exponentiation-operator.js b/lib/6to5/transformation/transformers/es7-exponentiation-operator.js
index 1310700c18..9ef16713d8 100644
--- a/lib/6to5/transformation/transformers/es7-exponentiation-operator.js
+++ b/lib/6to5/transformation/transformers/es7-exponentiation-operator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // https://github.com/rwaldron/exponentiation-operator
 
 exports.experimental = true;
diff --git a/lib/6to5/transformation/transformers/es7-generator-comprehension.js b/lib/6to5/transformation/transformers/es7-generator-comprehension.js
index 84d75634a1..cd229e5eb3 100644
--- a/lib/6to5/transformation/transformers/es7-generator-comprehension.js
+++ b/lib/6to5/transformation/transformers/es7-generator-comprehension.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var buildComprehension = require("../helpers/build-comprehension");
 var t                  = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/es7-object-spread.js b/lib/6to5/transformation/transformers/es7-object-spread.js
index 083bbdc166..87b32bff53 100644
--- a/lib/6to5/transformation/transformers/es7-object-spread.js
+++ b/lib/6to5/transformation/transformers/es7-object-spread.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // https://github.com/sebmarkbage/ecmascript-rest-spread
 
 var t = require("../../types");
diff --git a/lib/6to5/transformation/transformers/optional-async-to-generator.js b/lib/6to5/transformation/transformers/optional-async-to-generator.js
index d6f952783c..04f68f8cd9 100644
--- a/lib/6to5/transformation/transformers/optional-async-to-generator.js
+++ b/lib/6to5/transformation/transformers/optional-async-to-generator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var remapAsyncToGenerator = require("../helpers/remap-async-to-generator");
 var bluebirdCoroutines    = require("./optional-bluebird-coroutines");
 
diff --git a/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js b/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js
index 73b9db1fd7..1aa2a24863 100644
--- a/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js
+++ b/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var remapAsyncToGenerator = require("../helpers/remap-async-to-generator");
 var t                     = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/optional-core-aliasing.js b/lib/6to5/transformation/transformers/optional-core-aliasing.js
index d563bb619a..c0db8bd9bd 100644
--- a/lib/6to5/transformation/transformers/optional-core-aliasing.js
+++ b/lib/6to5/transformation/transformers/optional-core-aliasing.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var util     = require("../../util");
 var core     = require("core-js/library");
diff --git a/lib/6to5/transformation/transformers/optional-proto-to-assign.js b/lib/6to5/transformation/transformers/optional-proto-to-assign.js
index 73a9c8167d..1533480b76 100644
--- a/lib/6to5/transformation/transformers/optional-proto-to-assign.js
+++ b/lib/6to5/transformation/transformers/optional-proto-to-assign.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 var _ = require("lodash");
 
diff --git a/lib/6to5/transformation/transformers/optional-typeof-symbol.js b/lib/6to5/transformation/transformers/optional-typeof-symbol.js
index 1c1aed8cc1..a3d938ccf8 100644
--- a/lib/6to5/transformation/transformers/optional-typeof-symbol.js
+++ b/lib/6to5/transformation/transformers/optional-typeof-symbol.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.optional = true;
diff --git a/lib/6to5/transformation/transformers/optional-undefined-to-void.js b/lib/6to5/transformation/transformers/optional-undefined-to-void.js
index 19875cd90a..f233becf74 100644
--- a/lib/6to5/transformation/transformers/optional-undefined-to-void.js
+++ b/lib/6to5/transformation/transformers/optional-undefined-to-void.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.optional = true;
diff --git a/lib/6to5/transformation/transformers/playground-mallet-operator.js b/lib/6to5/transformation/transformers/playground-mallet-operator.js
index 559cbde1ed..5bd48d6046 100644
--- a/lib/6to5/transformation/transformers/playground-mallet-operator.js
+++ b/lib/6to5/transformation/transformers/playground-mallet-operator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var build = require("../helpers/build-conditional-assignment-operator-transformer");
 var t      = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/playground-memoization-operator.js b/lib/6to5/transformation/transformers/playground-memoization-operator.js
index 822692bcee..0f41ffb3c6 100644
--- a/lib/6to5/transformation/transformers/playground-memoization-operator.js
+++ b/lib/6to5/transformation/transformers/playground-memoization-operator.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var build = require("../helpers/build-conditional-assignment-operator-transformer");
 var t     = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/playground-method-binding.js b/lib/6to5/transformation/transformers/playground-method-binding.js
index 4f3fe3bb51..69e901f628 100644
--- a/lib/6to5/transformation/transformers/playground-method-binding.js
+++ b/lib/6to5/transformation/transformers/playground-method-binding.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.BindMemberExpression = function (node, parent, scope, context, file) {
diff --git a/lib/6to5/transformation/transformers/playground-object-getter-memoization.js b/lib/6to5/transformation/transformers/playground-object-getter-memoization.js
index 08af63ea57..d5acc71105 100644
--- a/lib/6to5/transformation/transformers/playground-object-getter-memoization.js
+++ b/lib/6to5/transformation/transformers/playground-object-getter-memoization.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../traverse");
 var t        = require("../../types");
 
diff --git a/lib/6to5/transformation/transformers/react.js b/lib/6to5/transformation/transformers/react.js
index a1ad20e23b..f906cead68 100644
--- a/lib/6to5/transformation/transformers/react.js
+++ b/lib/6to5/transformation/transformers/react.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // Based upon the excellent jsx-transpiler by Ingvar Stepanyan (RReverser)
 // https://github.com/RReverser/jsx-transpiler
 
diff --git a/lib/6to5/transformation/transformers/spec-block-scoped-functions.js b/lib/6to5/transformation/transformers/spec-block-scoped-functions.js
index 814937fd31..3d8d3449c8 100644
--- a/lib/6to5/transformation/transformers/spec-block-scoped-functions.js
+++ b/lib/6to5/transformation/transformers/spec-block-scoped-functions.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.FunctionDeclaration = function (node, parent) {
diff --git a/lib/6to5/transformation/transformers/spec-member-expression-literals.js b/lib/6to5/transformation/transformers/spec-member-expression-literals.js
index 6803865073..d4c318a996 100644
--- a/lib/6to5/transformation/transformers/spec-member-expression-literals.js
+++ b/lib/6to5/transformation/transformers/spec-member-expression-literals.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.MemberExpression = function (node) {
diff --git a/lib/6to5/transformation/transformers/spec-no-for-in-of-assignment.js b/lib/6to5/transformation/transformers/spec-no-for-in-of-assignment.js
index 0b00e88f9d..01de184aaa 100644
--- a/lib/6to5/transformation/transformers/spec-no-for-in-of-assignment.js
+++ b/lib/6to5/transformation/transformers/spec-no-for-in-of-assignment.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.ForInStatement =
diff --git a/lib/6to5/transformation/transformers/spec-property-literals.js b/lib/6to5/transformation/transformers/spec-property-literals.js
index f750fa7527..3473aa9f26 100644
--- a/lib/6to5/transformation/transformers/spec-property-literals.js
+++ b/lib/6to5/transformation/transformers/spec-property-literals.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var t = require("../../types");
 
 exports.Property = function (node) {
diff --git a/lib/6to5/transformation/transformers/spec-setters.js b/lib/6to5/transformation/transformers/spec-setters.js
index 54d88827aa..e104022bbc 100644
--- a/lib/6to5/transformation/transformers/spec-setters.js
+++ b/lib/6to5/transformation/transformers/spec-setters.js
@@ -1,3 +1,5 @@
+"use strict";
+
 exports.MethodDefinition =
 exports.Property = function (node, parent, scope, context, file) {
   if (node.kind === "set" && node.value.params.length !== 1) {
diff --git a/lib/6to5/transformation/transformers/use-strict.js b/lib/6to5/transformation/transformers/use-strict.js
index 41eec48c14..e8b813d253 100644
--- a/lib/6to5/transformation/transformers/use-strict.js
+++ b/lib/6to5/transformation/transformers/use-strict.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var useStrict = require("../helpers/use-strict");
 var t         = require("../../types");
 
diff --git a/lib/6to5/traverse/index.js b/lib/6to5/traverse/index.js
index 2e54a47505..3c88178c1b 100644
--- a/lib/6to5/traverse/index.js
+++ b/lib/6to5/traverse/index.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = traverse;
 
 /* jshint maxparams:7 */
diff --git a/lib/6to5/traverse/scope.js b/lib/6to5/traverse/scope.js
index e532848246..f100615f12 100644
--- a/lib/6to5/traverse/scope.js
+++ b/lib/6to5/traverse/scope.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Scope;
 
 var traverse = require("./index");
diff --git a/lib/6to5/types/index.js b/lib/6to5/types/index.js
index ff8af91e5e..31c783dc1f 100644
--- a/lib/6to5/types/index.js
+++ b/lib/6to5/types/index.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var toFastProperties = require("../to-fast-properties");
 var esutils          = require("esutils");
 var _                = require("lodash");
diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 626d06305d..40a2c23b97 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -1,3 +1,5 @@
+"use strict";
+
 require("./patch");
 
 var estraverse = require("estraverse");