Add Capability class.

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);
+    asyncEnd();
+  };
+}