Add runAsync interceptor.

tests/lib/async/intercept_run_async2_test.dart

+// Copyright (c) 2013, 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 "package:expect/expect.dart";
+import 'dart:async';
+
+var events = [];
+
+body() {
+  events.add("body entry");
+  runAsync(() {
+    events.add("run async body");
+  });
+  return 499;
+}
+
+handler(fun) {
+  events.add("handler");
+  fun();
+  events.add("handler done");
+}
+
+main() {
+  // Test that runAsync interception works.
+  var result = runZonedExperimental(body, onRunAsync: handler);
+  // No need for a ReceivePort: If the runZonedExperimental disbehaved we
+  // would have an [events] list that is different from what we expect.
+  Expect.listEquals(["body entry", "handler", "run async body", "handler done"],
+                    events);
+}