await in frog: partial support for try-catch blocks

frog/tests/await/src/await_test_helper.dart

Index: frog/tests/await/src/await_test_helper.dart
diff --git a/frog/tests/await/src/await_test_helper.dart b/frog/tests/await/src/await_test_helper.dart
index cc2d095e145a77646b1ffe246aeac4f7aafae7d0..1349382c5cc5e01bfc6d7c192c00570bfc99fa34 100644
--- a/frog/tests/await/src/await_test_helper.dart
+++ b/frog/tests/await/src/await_test_helper.dart
@@ -14,3 +14,10 @@ Future futureOf(value) {
   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;
+}