Add temporary directory for dartc compilation of tests.

frog/tests/await/test_config.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("await_test_config");
 
 #import("../../../tools/testing/dart/test_suite.dart");
 
 class AwaitTestSuite extends StandardTestSuite {
   AwaitTestSuite(Map configuration)
       : super(configuration,
               "await",
               "frog/tests/await/src",
               ["frog/tests/await/await.status"]);
 
-  List<String> additionalOptions() {
-    // Support running the tests from the frog as well as the
-    // top-level directory.
-    var awaitFile = new File("await/awaitc.dart");
-    if (!awaitFile.existsSync()) {
-      awaitFile = new File("frog/await/awaitc.dart");
-    }
-    return [awaitFile.fullPathSync()];
+  List<String> additionalOptions(String filename) {
+    return ['$dartDir/frog/await/awaitc.dart'];
   }
 }