Fixes from polymer.dart API review

pkg/polymer/test/prop_attr_reflection_test.dart

Index: pkg/polymer/test/prop_attr_reflection_test.dart
diff --git a/pkg/polymer/test/prop_attr_reflection_test.dart b/pkg/polymer/test/prop_attr_reflection_test.dart
index e429114ae1757107aba7a1105e96e393a8912aa1..37562dad192a8af0796e42b32274fc29ba2b9047 100644
--- a/pkg/polymer/test/prop_attr_reflection_test.dart
+++ b/pkg/polymer/test/prop_attr_reflection_test.dart
@@ -6,7 +6,6 @@ import 'dart:async';
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
-import 'package:polymer/platform.dart' as Platform;
 import 'package:polymer/polymer.dart';
 
 class XFoo extends PolymerElement {
@@ -37,7 +36,7 @@ Future onAttributeChange(Element node) {
     observer.disconnect();
     completer.complete();
   })..observe(node, attributes: true);
-  Platform.flush();
+  scheduleMicrotask(Observable.dirtyCheck);
   return completer.future;
 }