Fixes from polymer.dart API review

pkg/polymer/test/prop_attr_reflection_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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 {
   XFoo.created() : super.created();
 
   @observable var foo = '';
   @observable String baz = '';
 }
 
 class XBar extends XFoo {
@@ skipping 10 matching lines @@
 
   @observable bool zim = false;
 }
 
 Future onAttributeChange(Element node) {
   var completer = new Completer();
   new MutationObserver((records, observer) {
     observer.disconnect();
     completer.complete();
   })..observe(node, attributes: true);
-  Platform.flush();
+  scheduleMicrotask(Observable.dirtyCheck);
   return completer.future;
 }
 
 main() {
   initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   // Most tests use @CustomTag, here we test out the impertive register:
@@ skipping 60 matching lines @@
       expect(xbar.attributes.containsKey('zim'), false, reason:
           'attribute reflects false valued boolean property as NOT '
           'having attribute');
       xbar.obj = 'hi';
     }).then((_) => onAttributeChange(xbar)).then((_) {
       expect(xbar.attributes['obj'], 'hi', reason:
           'reflect property based on current type');
     });
   });
 }