diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index d4a0e6ef50..968796b731 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -262,7 +262,7 @@ File.prototype.transform = function (ast) {
   var self = this;
 
   this.ast = ast;
-  this.scope = new Scope(ast.program);
+  this.scope = new Scope(ast.program, null, this);
   this.moduleFormatter = this.getModuleFormatter(this.opts.modules);
 
   var astRun = function (key) {
diff --git a/lib/6to5/transformation/helpers/remap-async-to-generator.js b/lib/6to5/transformation/helpers/remap-async-to-generator.js
index 447ac18b76..d8473a6697 100644
--- a/lib/6to5/transformation/helpers/remap-async-to-generator.js
+++ b/lib/6to5/transformation/helpers/remap-async-to-generator.js
@@ -13,11 +13,11 @@ var traverser = {
   }
 };
 
-module.exports = function (node, callId) {
+module.exports = function (node, callId, scope) {
   node.async = false;
   node.generator = true;
 
-  traverse(node, traverser);
+  traverse(node, traverser, scope);
 
   var call = t.callExpression(callId, [node]);
 
diff --git a/lib/6to5/transformation/helpers/replace-supers.js b/lib/6to5/transformation/helpers/replace-supers.js
index 738cb6346a..addeb1261c 100644
--- a/lib/6to5/transformation/helpers/replace-supers.js
+++ b/lib/6to5/transformation/helpers/replace-supers.js
@@ -12,15 +12,17 @@ var t        = require("../../types");
  * @param {Object} className
  * @param {Object} superName
  * @param {Boolean} isLoose
+ * @param {Scope} scope
  * @param {File} file
  */
 
-function ReplaceSupers(methodNode, className, superName, isLoose, file) {
+function ReplaceSupers(methodNode, className, superName, isLoose, scope, file) {
   this.topLevelThisReference = null;
   this.methodNode            = methodNode;
   this.className             = className;
   this.superName             = superName;
   this.isLoose               = isLoose;
+  this.scope                 = scope;
   this.file                  = file;
 }
 
@@ -140,7 +142,7 @@ var traverser = {
 
 ReplaceSupers.prototype.traverseLevel = function (node, topLevel) {
   var state = { self: this, topLevel: topLevel };
-  traverse(node, traverser, null, state);
+  traverse(node, traverser, this.scope, state);
 };
 
 /**
diff --git a/lib/6to5/transformation/modules/_default.js b/lib/6to5/transformation/modules/_default.js
index 52ecda7cf8..079c020798 100644
--- a/lib/6to5/transformation/modules/_default.js
+++ b/lib/6to5/transformation/modules/_default.js
@@ -29,7 +29,7 @@ var exportsTraverser = {
 
 DefaultFormatter.prototype.getLocalExports = function () {
   var localExports = {};
-  traverse(this.file.ast, exportsTraverser, null, localExports);
+  traverse(this.file.ast, exportsTraverser, this.file.scope, localExports);
   return localExports;
 };
 
@@ -43,7 +43,7 @@ var importsTraverser = {
 
 DefaultFormatter.prototype.getLocalImports = function () {
   var localImports = {};
-  traverse(this.file.ast, importsTraverser, null, localImports);
+  traverse(this.file.ast, importsTraverser, this.file.scope, localImports);
   return localImports;
 };
 
@@ -140,7 +140,7 @@ var remapTraverser = {
 
 DefaultFormatter.prototype.remapAssignments = function () {
   var state = { self: this };
-  traverse(this.file.ast, remapTraverser, null, state);
+  traverse(this.file.ast, remapTraverser, this.file.scope, state);
 };
 
 DefaultFormatter.prototype.getModuleName = function () {
diff --git a/lib/6to5/transformation/modules/common.js b/lib/6to5/transformation/modules/common.js
index 00c4c84567..e1ee82fb84 100644
--- a/lib/6to5/transformation/modules/common.js
+++ b/lib/6to5/transformation/modules/common.js
@@ -18,7 +18,7 @@ function CommonJSFormatter(file) {
   DefaultFormatter.apply(this, arguments);
 
   var state = { hasNonDefaultExports: false };
-  traverse(file.ast, traverser, null, state);
+  traverse(file.ast, traverser, file.scope, state);
 
   this.hasNonDefaultExports = state.hasNonDefaultExports;
 }
diff --git a/lib/6to5/transformation/modules/system.js b/lib/6to5/transformation/modules/system.js
index 2f373a7d80..73be1a9444 100644
--- a/lib/6to5/transformation/modules/system.js
+++ b/lib/6to5/transformation/modules/system.js
@@ -83,6 +83,8 @@ var runnerSettersTraverser = {
 };
 
 SystemFormatter.prototype.buildRunnerSetters = function (block, hoistDeclarators) {
+  var scope = this.file.scope;
+
   return t.arrayExpression(_.map(this.ids, function (uid, source) {
     var state = {
       source:           source,
@@ -90,7 +92,7 @@ SystemFormatter.prototype.buildRunnerSetters = function (block, hoistDeclarators
       hoistDeclarators: hoistDeclarators
     };
 
-    traverse(block, runnerSettersTraverser, null, state);
+    traverse(block, runnerSettersTraverser, scope, state);
 
     return t.functionExpression(null, [uid], t.blockStatement(state.nodes));
   }));
@@ -171,7 +173,7 @@ SystemFormatter.prototype.transform = function (ast) {
   var returnStatement = handlerBody.pop();
 
   // hoist up all variable declarations
-  traverse(block, hoistVariablesTraverser, null, hoistDeclarators);
+  traverse(block, hoistVariablesTraverser, this.file.scope, hoistDeclarators);
 
   if (hoistDeclarators.length) {
     var hoistDeclar = t.variableDeclaration("var", hoistDeclarators);
@@ -180,7 +182,7 @@ SystemFormatter.prototype.transform = function (ast) {
   }
 
   // hoist up function declarations for circular references
-  traverse(block, hoistFunctionsTraverser, null, handlerBody);
+  traverse(block, hoistFunctionsTraverser, this.file.scope, handlerBody);
 
   handlerBody.push(returnStatement);
 
diff --git a/lib/6to5/transformation/transformer.js b/lib/6to5/transformation/transformer.js
index 63c6288dfd..65ba4792a3 100644
--- a/lib/6to5/transformation/transformer.js
+++ b/lib/6to5/transformation/transformer.js
@@ -76,7 +76,7 @@ Transformer.prototype.transform = function (file) {
   this.astRun(file, "before");
 
   var state = { file: file, transformer: this.transformer };
-  traverse(file.ast, transformTraverser, null, state);
+  traverse(file.ast, transformTraverser, file.scope, state);
 
   this.astRun(file, "after");
 };
diff --git a/lib/6to5/transformation/transformers/_alias-functions.js b/lib/6to5/transformation/transformers/_alias-functions.js
index 836bf8f362..74cdd54637 100644
--- a/lib/6to5/transformation/transformers/_alias-functions.js
+++ b/lib/6to5/transformation/transformers/_alias-functions.js
@@ -37,7 +37,7 @@ var functionTraverser = {
     }
 
     // traverse all child nodes of this function and find `arguments` and `this`
-    traverse(node, functionChildrenTraverser, null, state);
+    traverse(node, functionChildrenTraverser, scope, state);
 
     return context.skip();
   }
@@ -58,7 +58,7 @@ var go = function (getBody, node, file, scope) {
 
   // traverse the function and find all alias functions so we can alias
   // `arguments` and `this` if necessary
-  traverse(node, functionTraverser, null, state);
+  traverse(node, functionTraverser, scope, state);
 
   var body;
 
diff --git a/lib/6to5/transformation/transformers/es6/block-scoping.js b/lib/6to5/transformation/transformers/es6/block-scoping.js
index ef0e2c0c7f..694920f7bf 100644
--- a/lib/6to5/transformation/transformers/es6/block-scoping.js
+++ b/lib/6to5/transformation/transformers/es6/block-scoping.js
@@ -212,7 +212,7 @@ LetScoping.prototype.needsClosure = function () {
   var call = t.callExpression(fn, params);
   var ret  = this.file.generateUidIdentifier("ret", this.scope);
 
-  var hasYield = traverse.hasType(fn.body, "YieldExpression", t.FUNCTION_TYPES);
+  var hasYield = traverse.hasType(fn.body, this.scope, "YieldExpression", t.FUNCTION_TYPES);
   if (hasYield) {
     fn.generator = true;
     call = t.yieldExpression(call, true);
diff --git a/lib/6to5/transformation/transformers/es6/classes.js b/lib/6to5/transformation/transformers/es6/classes.js
index 807be19474..dd842f774a 100644
--- a/lib/6to5/transformation/transformers/es6/classes.js
+++ b/lib/6to5/transformation/transformers/es6/classes.js
@@ -137,7 +137,7 @@ Class.prototype.buildBody = function () {
   for (var i = 0; i < classBody.length; i++) {
     var node = classBody[i];
     if (t.isMethodDefinition(node)) {
-      var replaceSupers = new ReplaceSupers(node, this.className, this.superName, this.isLoose, this.file);
+      var replaceSupers = new ReplaceSupers(node, this.className, this.superName, this.isLoose, this.scope, this.file);
       replaceSupers.replace();
 
       if (node.key.name === "constructor") {
diff --git a/lib/6to5/transformation/transformers/es6/constants.js b/lib/6to5/transformation/transformers/es6/constants.js
index 701339c3d4..835f4bb4f0 100644
--- a/lib/6to5/transformation/transformers/es6/constants.js
+++ b/lib/6to5/transformation/transformers/es6/constants.js
@@ -83,5 +83,5 @@ exports.ForStatement = function (node, parent, scope, context, file) {
   if (!hasConstants) return;
 
   var state = { check: check, getIds: getIds };
-  traverse(node, traverser, null, state);
+  traverse(node, traverser, scope, state);
 };
diff --git a/lib/6to5/transformation/transformers/es7/comprehensions.js b/lib/6to5/transformation/transformers/es7/comprehensions.js
index e57dce69b5..9b9a4d2b9c 100644
--- a/lib/6to5/transformation/transformers/es7/comprehensions.js
+++ b/lib/6to5/transformation/transformers/es7/comprehensions.js
@@ -36,7 +36,7 @@ var array = function (node, parent, scope, file) {
   var block = container.callee.body;
   var body  = block.body;
 
-  if (traverse.hasType(node, "YieldExpression", t.FUNCTION_TYPES)) {
+  if (traverse.hasType(node, scope, "YieldExpression", t.FUNCTION_TYPES)) {
     container.callee.generator = true;
     container = t.yieldExpression(container, true);
   }
diff --git a/lib/6to5/transformation/transformers/optional/async-to-generator.js b/lib/6to5/transformation/transformers/optional/async-to-generator.js
index 017e7b3dd8..790078275a 100644
--- a/lib/6to5/transformation/transformers/optional/async-to-generator.js
+++ b/lib/6to5/transformation/transformers/optional/async-to-generator.js
@@ -10,5 +10,5 @@ exports.manipulateOptions = bluebirdCoroutines.manipulateOptions;
 exports.Function = function (node, parent, scope, context, file) {
   if (!node.async || node.generator) return;
 
-  return remapAsyncToGenerator(node, file.addHelper("async-to-generator"));
+  return remapAsyncToGenerator(node, file.addHelper("async-to-generator"), scope);
 };
diff --git a/lib/6to5/transformation/transformers/optional/bluebird-coroutines.js b/lib/6to5/transformation/transformers/optional/bluebird-coroutines.js
index 2057ce0b7d..3be41cdff6 100644
--- a/lib/6to5/transformation/transformers/optional/bluebird-coroutines.js
+++ b/lib/6to5/transformation/transformers/optional/bluebird-coroutines.js
@@ -16,6 +16,7 @@ exports.Function = function (node, parent, scope, context, file) {
 
   return remapAsyncToGenerator(
     node,
-    t.memberExpression(file.addImport("bluebird"), t.identifier("coroutine"))
+    t.memberExpression(file.addImport("bluebird"), t.identifier("coroutine")),
+    scope
   );
 };
diff --git a/lib/6to5/transformation/transformers/playground/object-getter-memoization.js b/lib/6to5/transformation/transformers/playground/object-getter-memoization.js
index 0287e12795..fa80f94d7a 100644
--- a/lib/6to5/transformation/transformers/playground/object-getter-memoization.js
+++ b/lib/6to5/transformation/transformers/playground/object-getter-memoization.js
@@ -36,5 +36,5 @@ exports.MethodDefinition = function (node, parent, scope, context, file) {
     file: file
   };
 
-  traverse(value, traverser, null, state);
+  traverse(value, traverser, scope, state);
 };
diff --git a/lib/6to5/traverse/index.js b/lib/6to5/traverse/index.js
index ee412284be..eddc67c249 100644
--- a/lib/6to5/traverse/index.js
+++ b/lib/6to5/traverse/index.js
@@ -164,6 +164,12 @@ function traverse(parent, opts, scope, state) {
   // falsy node
   if (!parent) return;
 
+  if (!scope) {
+    if (parent.type !== "Program" && parent.type !== "File") {
+      throw new Error("Must pass a scope unless traversing a Program/File got a " + parent.type + " node");
+    }
+  }
+
   if (!opts) opts = {};
   if (!opts.enter) opts.enter = _.noop;
   if (!opts.exit) opts.exit = _.noop;
@@ -217,7 +223,7 @@ function hasBlacklistedType(node, parent, scope, context, state) {
   }
 }
 
-traverse.hasType = function (tree, type, blacklistTypes) {
+traverse.hasType = function (tree, scope, type, blacklistTypes) {
   // the node we're searching in is blacklisted
   if (_.contains(blacklistTypes, tree.type)) return false;
 
@@ -232,7 +238,7 @@ traverse.hasType = function (tree, type, blacklistTypes) {
   traverse(tree, {
     blacklist: blacklistTypes,
     enter: hasBlacklistedType
-  }, null, state);
+  }, scope, state);
 
   return state.has;
 };
diff --git a/lib/6to5/traverse/scope.js b/lib/6to5/traverse/scope.js
index 434f387690..ddf685e412 100644
--- a/lib/6to5/traverse/scope.js
+++ b/lib/6to5/traverse/scope.js
@@ -14,11 +14,13 @@ var FOR_KEYS = ["left", "init"];
  *
  * @param {Node} block
  * @param {Scope} [parent]
+ * @param {File} [file]
  */
 
-function Scope(block, parent) {
+function Scope(block, parent, file) {
   this.parent = parent;
   this.block  = block;
+  this.file   = parent ? parent.file : file;
 
   var info = this.getInfo();
   this.references = info.references;
@@ -34,7 +36,7 @@ Scope.defaultDeclarations = _.flatten([
   vars.reservedVars
 ].map(_.keys));
 
-Scope.add = function (node, references) {
+Scope.add = function (node, references, throwOnDuplicate) {
   if (!node) return;
   _.defaults(references, t.getIds(node, true));
 };