Fix tests for VM in checked mode.

runtime/tests/vm/dart/isolate_unhandled_exception_test.dart

 // Copyright (c) 2012, 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.
 
 library isolate_unhandled_exception_test;
 
+import 'dart:async';
 import 'dart:isolate';
 
 // Tests that an isolate's keeps message handling working after
 // throwing an unhandled exception, if it was created with an
 // unhandled exception callback that returns true (continue handling).
 // This test verifies that a callback function specified in
 // Isolate.spawnFunction is called.
 
 // Note: this test will hang if an uncaught exception isn't handled,
 // either by an error in the callback or it returning false.
@@ skipping 23 matching lines @@
     Expect.fail("Error not expected");
   });
 
   // Verify that isolate can still handle messages.
   isolate_port.call('hi').then((value) {
     Expect.equals('hello', value);
   }, onError: (error) {
     Expect.fail("Error not expected");
   });
 }