[modules] Make @@toStringTag on namespace objects non-configurable.

test/mjsunit/modules-namespace1.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
 
 let ja = 42;
 export {ja as yo};
 export const bla = "blaa";
 export {foo as foo_again};
@@ skipping 28 matching lines @@
 assertEquals(42, Reflect.get(foo, "yo"));
 assertEquals(43, (ja++, foo.yo));
 
 // Its "foo_again" property.
 assertSame(foo, foo.foo_again);
 
 // Its @@toStringTag property.
 assertTrue(Reflect.has(foo, Symbol.toStringTag));
 assertEquals("string", typeof Reflect.get(foo, Symbol.toStringTag));
 assertEquals(
-    {value: "Module", configurable: true, writable: false, enumerable: false},
+    {value: "Module", configurable: false, writable: false, enumerable: false},
     Reflect.getOwnPropertyDescriptor(foo, Symbol.toStringTag));
-
-// TODO(neis): Clarify spec w.r.t. other symbols.
+// TODO(neis): Spec currently says the next one should return true.
+assertFalse(Reflect.deleteProperty(foo, Symbol.toStringTag));
 
 // Nonexistant properties.
 let nonexistant = ["gaga", 123, Symbol('')];
 for (let key of nonexistant) {
   assertSame(undefined, Reflect.getOwnPropertyDescriptor(foo, key));
   assertTrue(Reflect.deleteProperty(foo, key));
   assertFalse(Reflect.set(foo, key, true));
   assertSame(undefined, Reflect.get(foo, key));
   assertFalse(Reflect.defineProperty(foo, key, {get() {return 1}}));
   assertFalse(Reflect.has(foo, key));
 }
 
 // The actual star import that we are testing. Namespace imports are
 // initialized before evaluation.
 import * as foo from "modules-namespace1.js";
 
 // There can be only one namespace object.
 import * as bar from "modules-namespace1.js";
 assertSame(foo, bar);