Add expectAsyncX and expectAsyncUntilX methods, and deprecate the old methods.

test/runner/signal_test.dart

Index: test/runner/signal_test.dart
diff --git a/test/runner/signal_test.dart b/test/runner/signal_test.dart
index 42c99c2b2b80038c3b2db0dbeef68ee83087eb93..bc6c054d01e376ddd10d883bda7bd7cfe1a90595 100644
--- a/test/runner/signal_test.dart
+++ b/test/runner/signal_test.dart
@@ -234,7 +234,7 @@ void main() {
 
     await new Future.delayed(new Duration(seconds: 1));
     try {
-      expectAsync(() {});
+      expectAsync0(() {});
     } catch (_) {
       expectAsyncThrewError = true;
     }