Add runAsync interceptor.

tests/lib/async/intercept_run_async1_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';
 
 main() {
-  // Make sure `runZoned` returns the result of a synchronous call.
-  Expect.equals(499, runZonedExperimental(() => 499));
+  // Test that runZoned returns the result of executing the body.
+  var result = runZonedExperimental(() => 499,
+    onRunAsync: (f) {
+      Expect.fail("Unexpected invocation.");
+    });
+  Expect.equals(499, result);
 }