diff --git a/lib/6to5/transformation/templates/corejs-iterator.js b/lib/6to5/transformation/templates/corejs-iterator.js
new file mode 100644
index 0000000000..c30ea8a4c2
--- /dev/null
+++ b/lib/6to5/transformation/templates/corejs-iterator.js
@@ -0,0 +1 @@
+CORE_ID.$for.getIterator(VALUE);
diff --git a/lib/6to5/transformation/transform.js b/lib/6to5/transformation/transform.js
index d9fa1c90b4..e40af5089f 100644
--- a/lib/6to5/transformation/transform.js
+++ b/lib/6to5/transformation/transform.js
@@ -80,6 +80,8 @@ _.each({
 
   _declarations:             require("./transformers/_declarations"),
 
+  coreAliasing:              require("./transformers/optional-core-aliasing"),
+
   // wrap up
   _aliasFunctions:           require("./transformers/_alias-functions"),
   useStrict:                 require("./transformers/use-strict"),
diff --git a/lib/6to5/transformation/transformer.js b/lib/6to5/transformation/transformer.js
index 675b2743f2..92b1082836 100644
--- a/lib/6to5/transformation/transformer.js
+++ b/lib/6to5/transformation/transformer.js
@@ -5,9 +5,11 @@ var t        = require("../types");
 var _        = require("lodash");
 
 function Transformer(key, transformer, opts) {
-  this.transformer = Transformer.normalise(transformer);
-  this.opts        = opts || {};
-  this.key         = key;
+  this.experimental = !!transformer.experimental;
+  this.transformer  = Transformer.normalise(transformer);
+  this.optional     = !!transformer.optional;
+  this.opts         = opts || {};
+  this.key          = key;
 }
 
 Transformer.normalise = function (transformer) {
@@ -16,8 +18,13 @@ Transformer.normalise = function (transformer) {
   }
 
   _.each(transformer, function (fns, type) {
+    // hidden property
     if (type[0] === "_") return;
+
     if (_.isFunction(fns)) fns = { enter: fns };
+
+    if (!_.isObject(fns)) return;
+
     transformer[type] = fns;
 
     var aliases = t.FLIPPED_ALIAS_KEYS[type];
@@ -41,13 +48,9 @@ Transformer.prototype.astRun = function (file, key) {
 };
 
 Transformer.prototype.transform = function (file) {
-  if (!this.canRun(file)) return;
-
   var transformer = this.transformer;
   var ast         = file.ast;
 
-  this.astRun(file, "enter");
-
   var build = function (exit) {
     return function (node, parent, scope) {
       var fns = transformer[node.type];
@@ -61,12 +64,14 @@ Transformer.prototype.transform = function (file) {
     };
   };
 
+  this.astRun(file, "before");
+
   traverse(ast, {
     enter: build(),
     exit:  build(true)
   });
 
-  this.astRun(file, "exit");
+  this.astRun(file, "after");
 };
 
 Transformer.prototype.canRun = function (file) {
@@ -80,5 +85,9 @@ Transformer.prototype.canRun = function (file) {
   var whitelist = opts.whitelist;
   if (whitelist.length && !_.contains(whitelist, key)) return false;
 
+  if (this.optional && !_.contains(opts.optional, key)) return false;
+
+  if (this.experimental && !opts.experimental) return false;
+
   return true;
 };
diff --git a/lib/6to5/types/builder-keys.json b/lib/6to5/types/builder-keys.json
index beca94c0e4..e9ac7987be 100644
--- a/lib/6to5/types/builder-keys.json
+++ b/lib/6to5/types/builder-keys.json
@@ -11,6 +11,8 @@
   "FunctionExpression":      ["id", "params", "body", "generator"],
   "Identifier":              ["name"],
   "IfStatement":             ["test", "consequent", "alternate"],
+  "ImportDeclaration":       ["specifiers", "source"],
+  "ImportSpecifier":         ["id", "name"],
   "Literal":                 ["value"],
   "LogicalExpression":       ["operator", "left", "right"],
   "MemberExpression":        ["object", "property", "computed"],
diff --git a/test/fixtures/transformation/optional-core-aliasing/es6-destructuring/actual.js b/test/fixtures/transformation/optional-core-aliasing/es6-destructuring/actual.js
new file mode 100644
index 0000000000..10aa8799ed
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es6-destructuring/actual.js
@@ -0,0 +1 @@
+var [a, [b], [c], d] = ["hello", [", ", "junk"], ["world"]];
diff --git a/test/fixtures/transformation/optional-core-aliasing/es6-destructuring/expected.js b/test/fixtures/transformation/optional-core-aliasing/es6-destructuring/expected.js
new file mode 100644
index 0000000000..712e62b592
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es6-destructuring/expected.js
@@ -0,0 +1,35 @@
+"use strict";
+
+var _slicedToArray = function (arr, i) {
+  if (Array.isArray(arr)) {
+    return arr;
+  } else {
+    var _arr = [];
+    for (var _iterator = _core.$for.getIterator(arr), _step; !(_step = _iterator.next()).done;) {
+      _arr.push(_step.value);
+
+      if (i && _arr.length === i) break;
+    }
+
+    return _arr;
+  }
+};
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var _ref = ["hello", [", ", "junk"], ["world"]];
+
+var _ref2 = _slicedToArray(_ref, 4);
+
+var a = _ref2[0];
+var _ref3 = _slicedToArray(_ref2[1], 1);
+
+var b = _ref3[0];
+var _ref4 = _slicedToArray(_ref2[2], 1);
+
+var c = _ref4[0];
+var d = _ref2[3];
diff --git a/test/fixtures/transformation/optional-core-aliasing/es6-for-of/actual.js b/test/fixtures/transformation/optional-core-aliasing/es6-for-of/actual.js
new file mode 100644
index 0000000000..48e5f59b2c
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es6-for-of/actual.js
@@ -0,0 +1,3 @@
+for (var i of arr) {
+
+}
diff --git a/test/fixtures/transformation/optional-core-aliasing/es6-for-of/expected.js b/test/fixtures/transformation/optional-core-aliasing/es6-for-of/expected.js
new file mode 100644
index 0000000000..3bf1f53a05
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es6-for-of/expected.js
@@ -0,0 +1,11 @@
+"use strict";
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+for (var _iterator = _core.$for.getIterator(arr), _step; !(_step = _iterator.next()).done;) {
+  var i = _step.value;
+}
diff --git a/test/fixtures/transformation/optional-core-aliasing/es6-spread/actual.js b/test/fixtures/transformation/optional-core-aliasing/es6-spread/actual.js
new file mode 100644
index 0000000000..5ebb80da66
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es6-spread/actual.js
@@ -0,0 +1 @@
+var lyrics = ["head", "and", "toes", ...parts];
diff --git a/test/fixtures/transformation/optional-core-aliasing/es6-spread/expected.js b/test/fixtures/transformation/optional-core-aliasing/es6-spread/expected.js
new file mode 100644
index 0000000000..4d00347582
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es6-spread/expected.js
@@ -0,0 +1,13 @@
+"use strict";
+
+var _toArray = function (arr) {
+  return Array.isArray(arr) ? arr : _core.Array.from(arr);
+};
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var lyrics = ["head", "and", "toes"].concat(_toArray(parts));
diff --git a/test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/actual.js b/test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/actual.js
new file mode 100644
index 0000000000..9dda19048b
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/actual.js
@@ -0,0 +1 @@
+var arr = [for (i of nums) i * i];
diff --git a/test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/expected.js b/test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/expected.js
new file mode 100644
index 0000000000..ccfa596f71
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/expected.js
@@ -0,0 +1,15 @@
+"use strict";
+
+var _toArray = function (arr) {
+  return Array.isArray(arr) ? arr : _core.Array.from(arr);
+};
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var arr = _toArray(nums).map(function (i) {
+  return i * i;
+});
diff --git a/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/actual.js b/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/actual.js
new file mode 100644
index 0000000000..a16dd23cb9
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/actual.js
@@ -0,0 +1 @@
+var z = { ...x };
diff --git a/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/expected.js b/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/expected.js
new file mode 100644
index 0000000000..940d6e9d13
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/expected.js
@@ -0,0 +1,9 @@
+"use strict";
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var z = _core.Object.assign({}, x);
diff --git a/test/fixtures/transformation/optional-core-aliasing/options.json b/test/fixtures/transformation/optional-core-aliasing/options.json
new file mode 100644
index 0000000000..371b5f0b20
--- /dev/null
+++ b/test/fixtures/transformation/optional-core-aliasing/options.json
@@ -0,0 +1,4 @@
+{
+  "optional": ["coreAliasing"],
+  "experimental": true
+}