Fix dart2js by copying keys of a map that is modified under us.

tests/isolate/timer_not_available_test.dart

 // Copyright (c) 2011, 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 timerNotAvailable;
 
-import 'dart:isolate';
+import 'dart:async';
 
 main() {
   bool failed = false;
   try {
     new Timer(0, (_) { });
   } on UnsupportedError catch (e) {
     failed = true;
   }
   Expect.isTrue(failed);
   failed = false;
   try {
     var t = new Timer.repeating(10, (_) { });
     t.cancel();
   } on UnsupportedError catch (e) {
     failed = true;
   }
   Expect.isTrue(failed);
 }