fix dart2js warnings in polymer_expressions pkg

pkg/polymer_expressions/test/globals_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:mdv/mdv.dart' as mdv;
 import 'package:observe/observe.dart';
 import 'package:observe/src/microtask.dart';
 import 'package:polymer_expressions/polymer_expressions.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_enhanced_config.dart';
 
 main() {
@@ skipping 29 matching lines @@
 
       expect(testDiv.queryAll('div').map((n) => n.text), [
         'Item 0 is hello',
         'Item 1 is from',
         'Item 2 is polymer',
         'Item 3 is expressions',
       ]);
     }));
   });
 }