Fix test_runner_test which uses Timer to import dart:async.

tests/standalone/io/test_runner_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.
 
 import "dart:io";
 import "dart:isolate";
+import "dart:async";
 import "../../../tools/testing/dart/test_runner.dart";
 import "../../../tools/testing/dart/status_file_parser.dart";
 import "../../../tools/testing/dart/test_options.dart";
 import "process_test_util.dart";
 
 class TestController {
   static const int numTests = 4;
   static int numCompletedTests = 0;
 
   // Used as TestCase.completedCallback.
@@ skipping 69 matching lines @@
         break;
       case 'timeout':
         // Run for 10 seconds, then exit.  This tests a 2 second timeout.
         new Timer(10 * 1000, (t){ });
         break;
       default:
         throw "Unknown option ${arguments[0]} passed to test_runner_test";
     }
   }
 }