Add temporary directory for dartc compilation of tests.

frog/tests/await/test_config.dart

Index: frog/tests/await/test_config.dart
diff --git a/frog/tests/await/test_config.dart b/frog/tests/await/test_config.dart
index 4447bcb57710b935818677fa99d8a7e4f6488186..4de5ebbef0eb580a2d762bf5cb3e7c6200c63a05 100644
--- a/frog/tests/await/test_config.dart
+++ b/frog/tests/await/test_config.dart
@@ -13,13 +13,7 @@ class AwaitTestSuite extends StandardTestSuite {
               "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'];
   }
 }