add extends helper instead of using Object.assign

lib/6to5/file.js

@@ -35,7 +35,8 @@ File.helpers = [
   "async-to-generator",
   "interop-require-wildcard",
   "typeof",
-  "exports-wildcard"
+  "exports-wildcard",
+  "extends"
 ];
 
 File.excludeHelpersFromRuntime = [

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);

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);

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;
+})

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);
 };

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);
   }
 };