First pass at asynchronous input loading in dart2js.

tests/async_helper.dart

Index: tests/async_helper.dart
diff --git a/tests/async_helper.dart b/tests/async_helper.dart
index f1a84b369f782285c87d0250fa5ef5ece290a8b3..72a391f03a019a3c5785011d3ba5141ec03ca68d 100644
--- a/tests/async_helper.dart
+++ b/tests/async_helper.dart
@@ -61,3 +61,8 @@ void asyncEnd() {
     print('unittest-suite-success');
   }
 }
+
+void asyncTest(Future f()) {
+  asyncStart();
+  f().whenComplete(() => asyncEnd());
+}