[polymer] fix editor hints -- query/queryAll and unused imports

pkg/polymer/test/attr_mustache_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:polymer/polymer.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:template_binding/template_binding.dart';
@@ skipping 33 matching lines @@
   @observable var src = 'testSource';
 }
 
 main() {
   initPolymer();
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('mustache attributes', () {
-    final xtest = document.query('#test');
-    final xtarget = xtest.shadowRoot.query('#target');
+    final xtest = document.querySelector('#test');
+    final xtarget = xtest.shadowRoot.querySelector('#target');
     return xtarget.foundSrc;
   });
 }