Add Capability implementation to dart2js libraries.

tests/isolate/capability_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:isolate";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 void main() {
   asyncStart();
   var c1 = new Capability();
   var c2 = new Capability();
-  Expect.notIdentical(c1, c2);
   Expect.notEquals(c1, c2);
 
   var receive = new RawReceivePort();
   receive.sendPort.send(c1);
   receive.handler = (c3) {
-    Expect.identical(c3, c1);
+    Expect.equals(c3, c1);
+    Expect.notEquals(c3, c2);
     asyncEnd();
   };
 }