diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index f222387830..da5b168fa3 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -35,7 +35,8 @@ File.helpers = [
   "async-to-generator",
   "interop-require-wildcard",
   "typeof",
-  "exports-wildcard"
+  "exports-wildcard",
+  "extends"
 ];
 
 File.excludeHelpersFromRuntime = [
diff --git a/lib/6to5/transformation/modules/common.js b/lib/6to5/transformation/modules/common.js
index 7364d6a3fd..7fc557d6c0 100644
--- a/lib/6to5/transformation/modules/common.js
+++ b/lib/6to5/transformation/modules/common.js
@@ -83,10 +83,12 @@ CommonJSFormatter.prototype.exportDeclaration = function (node, nodes) {
       // this export isn't a function so we can't hoist it to the top so we need to set it
       // at the very end of the file with something like:
       //
-      //   module.exports = Object.assign(exports["default"], exports)
+      //   module.exports = _extends(exports["default"], exports)
       //
 
-      assign = util.template("common-export-default-assign", true);
+      assign = util.template("common-export-default-assign", {
+        EXTENDS_HELPER: this.file.addHelper("extends")
+      }, true);
       assign._blockHoist = 0;
 
       nodes.push(assign);
diff --git a/lib/6to5/transformation/templates/common-export-default-assign.js b/lib/6to5/transformation/templates/common-export-default-assign.js
index 17a1f96968..557c221edb 100644
--- a/lib/6to5/transformation/templates/common-export-default-assign.js
+++ b/lib/6to5/transformation/templates/common-export-default-assign.js
@@ -1 +1 @@
-module.exports = Object.assign(exports["default"], exports);
+module.exports = EXTENDS_HELPER(exports["default"], exports);
diff --git a/lib/6to5/transformation/templates/extends.js b/lib/6to5/transformation/templates/extends.js
new file mode 100644
index 0000000000..b75e9a76a9
--- /dev/null
+++ b/lib/6to5/transformation/templates/extends.js
@@ -0,0 +1,9 @@
+(function (target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      target[key] = source[key];
+    }
+  }
+  return target;
+})
diff --git a/lib/6to5/transformation/transformers/es7-object-spread.js b/lib/6to5/transformation/transformers/es7-object-spread.js
index 47469a35bc..81156bfb1f 100644
--- a/lib/6to5/transformation/transformers/es7-object-spread.js
+++ b/lib/6to5/transformation/transformers/es7-object-spread.js
@@ -4,7 +4,7 @@ var t = require("../../types");
 
 exports.experimental = true;
 
-exports.ObjectExpression = function (node) {
+exports.ObjectExpression = function (node, parent, file) {
   var hasSpread = false;
   var i;
   var prop;
@@ -42,5 +42,5 @@ exports.ObjectExpression = function (node) {
     args.unshift(t.objectExpression([]));
   }
 
-  return t.callExpression(t.memberExpression(t.identifier("Object"), t.identifier("assign")), args);
+  return t.callExpression(file.addHelper("extends"), args);
 };
diff --git a/lib/6to5/transformation/transformers/optional-proto-to-assign.js b/lib/6to5/transformation/transformers/optional-proto-to-assign.js
index 5de64efb34..6622193759 100644
--- a/lib/6to5/transformation/transformers/optional-proto-to-assign.js
+++ b/lib/6to5/transformation/transformers/optional-proto-to-assign.js
@@ -43,7 +43,7 @@ exports.ExpressionStatement = function (node, parent, file) {
   }
 };
 
-exports.ObjectExpression = function (node) {
+exports.ObjectExpression = function (node, parent, file) {
   var proto;
 
   for (var i in node.properties) {
@@ -58,6 +58,6 @@ exports.ObjectExpression = function (node) {
   if (proto) {
     var args = [t.objectExpression([]), proto];
     if (node.properties.length) args.push(node);
-    return t.callExpression(OBJECT_ASSIGN_MEMBER, args);
+    return t.callExpression(file.addHelper("extends"), args);
   }
 };
diff --git a/test/fixtures/transformation/es6-modules-common/exports-default-non-function/expected.js b/test/fixtures/transformation/es6-modules-common/exports-default-non-function/expected.js
index 8922e90b48..a8d178e952 100644
--- a/test/fixtures/transformation/es6-modules-common/exports-default-non-function/expected.js
+++ b/test/fixtures/transformation/es6-modules-common/exports-default-non-function/expected.js
@@ -1,6 +1,17 @@
 "use strict";
 
+var _extends = function (target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      target[key] = source[key];
+    }
+  }
+
+  return target;
+};
+
 exports.Cachier = Cachier;
 exports["default"] = new Cachier();
 function Cachier(databaseName) {}
-module.exports = Object.assign(exports["default"], exports);
+module.exports = _extends(exports["default"], exports);
diff --git a/test/fixtures/transformation/es7-object-spread/assignment/expected.js b/test/fixtures/transformation/es7-object-spread/assignment/expected.js
index bd7dab2734..5671a88356 100644
--- a/test/fixtures/transformation/es7-object-spread/assignment/expected.js
+++ b/test/fixtures/transformation/es7-object-spread/assignment/expected.js
@@ -1,3 +1,14 @@
 "use strict";
 
-z = Object.assign({ x: x }, y);
+var _extends = function (target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      target[key] = source[key];
+    }
+  }
+
+  return target;
+};
+
+z = _extends({ x: x }, y);
diff --git a/test/fixtures/transformation/es7-object-spread/expression/expected.js b/test/fixtures/transformation/es7-object-spread/expression/expected.js
index 856b5c1f8c..6080159f0f 100644
--- a/test/fixtures/transformation/es7-object-spread/expression/expected.js
+++ b/test/fixtures/transformation/es7-object-spread/expression/expected.js
@@ -1,3 +1,14 @@
 "use strict";
 
-Object.assign({ x: x }, y, { a: a }, b, { c: c });
+var _extends = function (target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      target[key] = source[key];
+    }
+  }
+
+  return target;
+};
+
+_extends({ x: x }, y, { a: a }, b, { c: c });
diff --git a/test/fixtures/transformation/es7-object-spread/variable-declaration/expected.js b/test/fixtures/transformation/es7-object-spread/variable-declaration/expected.js
index 2522635156..8fee368e87 100644
--- a/test/fixtures/transformation/es7-object-spread/variable-declaration/expected.js
+++ b/test/fixtures/transformation/es7-object-spread/variable-declaration/expected.js
@@ -1,3 +1,14 @@
 "use strict";
 
-var z = Object.assign({}, x);
+var _extends = function (target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      target[key] = source[key];
+    }
+  }
+
+  return target;
+};
+
+var z = _extends({}, x);
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
deleted file mode 100644
index a16dd23cb9..0000000000
--- a/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/actual.js
+++ /dev/null
@@ -1 +0,0 @@
-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
deleted file mode 100644
index 940d6e9d13..0000000000
--- a/test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/expected.js
+++ /dev/null
@@ -1,9 +0,0 @@
-"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-proto-to-assign/object-literal/expected.js b/test/fixtures/transformation/optional-proto-to-assign/object-literal/expected.js
index 31b92ba185..49087a66e7 100644
--- a/test/fixtures/transformation/optional-proto-to-assign/object-literal/expected.js
+++ b/test/fixtures/transformation/optional-proto-to-assign/object-literal/expected.js
@@ -1,10 +1,21 @@
 "use strict";
 
-var foo = Object.assign({}, bar);
+var _extends = function (target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      target[key] = source[key];
+    }
+  }
 
-var foo = Object.assign({}, bar, {
+  return target;
+};
+
+var foo = _extends({}, bar);
+
+var foo = _extends({}, bar, {
   bar: "foo" });
 
-var foo = Object.assign({}, bar, {
+var foo = _extends({}, bar, {
   bar: "foo"
 });