Add a test for subclass maps.

test/mjsunit/harmony/classes-maps.js

+// Copyright 2014 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.
+//
+// Flags: --harmony-classes --allow-natives-syntax
+'use strict';
+
+(function TestMaps() {
+  class Base {}
+  class Derived extends Base {}
+
+  let d1 = new Derived();
+  let d2 = new Derived();
+
+  assertTrue(%HaveSameMap(d1, d2));
+}());
+
+
+(function TestProtoModificationArray() {
+  let called = 0;
+  function F() {
+    called++;
+    assertFalse(Array.isArray(this));
+  }
+  class Derived extends Array {}
+  assertSame(Derived.__proto__, Array);
+
+  let d1 = new Derived();
+  assertTrue(Array.isArray(d1));
+
+  Derived.__proto__ = F;
+  called = 0;
+  let d2 = new Derived();
+  assertSame(1, called);
+  assertFalse(Array.isArray(d2));
+
+  assertFalse(%HaveSameMap(d1, d2));
+}());
+
+
+(function TestProtoModification() {
+  let called = 0;
+  function F() {
+    called++;
+    let exn = null;
+    try {
+      this.byteLength;
+    }

[arv (Not doing code reviews), 2015/04/13 15:59:39]

nit: 

} catch (e) {

[Dmitry Lomov (no reviews), 2015/04/14 09:34:27]

Done.

+    catch (e) {
+      exn = e;
+    }
+    assertTrue(exn instanceof TypeError);
+  }
+  class Derived extends Uint8Array {
+    constructor() { super(10); }
+  }
+  assertSame(Derived.__proto__, Uint8Array);
+
+  let d1 = new Derived();
+  assertSame(10, d1.byteLength);
+
+  Derived.__proto__ = F;
+  called = 0;
+  let d2 = new Derived();
+  assertSame(1, called);
+  assertThrows(function() { d2.byteLength; }, TypeError);
+
+  assertFalse(%HaveSameMap(d1, d2));
+}());