[modules] Implement @@iterator on namespace objects.

test/mjsunit/modules-namespace2.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // MODULE
 
-// TODO(neis): Symbol.iterator
 assertEquals(
-    ["b", "c", "get_a", "ns2", "set_a", "zzz", Symbol.toStringTag],
+    ["b", "c", "get_a", "ns2", "set_a", "zzz",
+     Symbol.toStringTag, Symbol.iterator],
     Reflect.ownKeys(ns));
-// assertEquals(["b", "c", "get_a", "ns2", "set_a", "zzz"], [...ns]);
+assertEquals(["b", "c", "get_a", "ns2", "set_a", "zzz"], [...ns]);
 
 import * as foo from "modules-skip-1.js";
 assertSame(foo.a, ns.b);
 assertSame(foo.a, ns.c);
 assertSame(foo.get_a, ns.get_a);
 assertSame(foo.set_a, ns.set_a);
 assertEquals(123, ns.zzz);
 
 assertSame(ns, ns.ns2.ns);
 import * as ns from "modules-skip-namespace.js";
 export {ns};