Improve Object.assign

- Use Object.keys (now fixed!) to obtain list of enumerable own keys,
  and iterate over that rather than using a for-in loop.  This avoids
  having to check whether the keys are own keys or not.

- Make Object.assign non-enumerable.

- Add tests.

server/startup/es6.js

@@ -91,24 +91,25 @@ var WeakMap = new 'WeakMap';
 ///////////////////////////////////////////////////////////////////////////////
 
 Object.assign = function assign(target, varArgs) {
-  // Polyfill adapted from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#polyfill
   // The length property of the assign method is 2.
   if (target === null || target === undefined) {
     throw new TypeError('Cannot convert undefined or null to object');
   }
-  for (var index = 1; index < arguments.length; index++) {
-    var nextSource = arguments[index];
-    if (nextSource !== null && nextSource !== undefined) {
-      for (var nextKey in nextSource) {
-        // Avoid bugs when hasOwnProperty is shadowed
-        if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
-          to[nextKey] = nextSource[nextKey];
-        }
+  target = Object(target);
+
+  for (var i = 1; i < arguments.length; i++) {
+    var src = arguments[i];
+    if (src !== null && src !== undefined) {
+      var keys = Object.keys(src);
+      for (var j = 0; j < keys.length; j++) {
+        var key = keys[j];
+        target[key] = src[key];
       }
     }
   }
-  return to;
+  return target;
 };
+Object.defineProperty(Array, 'assign', {enumerable: false});
 
 
 ///////////////////////////////////////////////////////////////////////////////

server/tests/testcases.js

@@ -1476,6 +1476,26 @@ module.exports = [
     `,
     expected: 82,
   },
+  {
+    name: 'Object.assign',
+    src: `
+      var p = {x: 'inherited enumerable', y: 'inherited nonenumerable'};
+      var o = Object.create(p);
+      o.a = 'own enumerable';
+      o.b = 'own nonenumerable';
+      o.c = 'own enumerable';
+      Object.defineProperty(p, 'y', {enumerable: false});
+      Object.defineProperty(o, 'b', {enumerable: false});
+
+      var t = {a: 'to be overwritten', b: 'not overwritten', d: 'preserved'};
+
+      Object.assign(t, o, {e: 'extra'});
+      [Object.getOwnPropertyNames(t).length,
+       t.a, t.b, t.c, t.d, t.e].toString();
+    `,
+    expected:
+      '5,own enumerable,not overwritten,own enumerable,preserved,extra',
+  },
   {
     name: 'Object.getOwnPropertyNames',
     src: `