await in frog: partial support for try-catch blocks

frog/tests/await/src/await_test_helper.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.
 
 // A library used by await tests.
 #library("await_test_helper.dart");
 
 #import("../../../lib/node/node.dart");
 
 /** Returns a future that completes with [value] after calling [setTimeout]. */
 Future futureOf(value) {
   final c = new Completer();
   final f = c.future;
   setTimeout(() { c.complete(value); }, 0);
   return f;
 }
+
+Future errorOf(error) {
+  final c = new Completer();
+  final f = c.future;
+  setTimeout(() { c.completeException(error); }, 0);
+  return f;
+}