Fixes from polymer.dart API review

pkg/polymer/lib/platform.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.
-
-/**
- * Exposes helper functionality for interacting with the platform. Similar to
- * the [Platform] class from dart:html.
- */
-// TODO(jmesserly): in Polymer this is a static class called "Platform", but
-// that conflicts with dart:html. What should we do? Does this functionality
-// belong in html's Platform instead?
-library polymer.platform;
-
-import 'dart:async' show Completer;
-import 'dart:html' show Text, MutationObserver;
-import 'dart:collection' show Queue;
-import 'package:observe/observe.dart';
-
-void flush() => endOfMicrotask(Observable.dirtyCheck);
-
-int _iterations = 0;
-final Queue _callbacks = new Queue();
-final Text _twiddle = () {
-  var twiddle = new Text('');
-  new MutationObserver((x, y) {
-    while (_callbacks.isNotEmpty) {
-      try {
-        _callbacks.removeFirst()();
-      } catch (e) { // Dart note: fire the error async.
-        new Completer().completeError(e);
-      }
-    }
-  }).observe(twiddle, characterData: true);
-  return twiddle;
-}();
-
-void endOfMicrotask(void callback()) {
-  _twiddle.text = '${_iterations++}';
-  _callbacks.add(callback);
-}