Fix warnings and hints.

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/src/microtask.dart' show performMicrotaskCheckpoint;
 
 void flush() {
   endOfMicrotask(performMicrotaskCheckpoint);
 }
 
 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);
 }