Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/output_collector.dart

 // Copyright (c) 2015, 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.
 
 // Output provider that collects the output in string buffers.
 
 library output_collector;
 
-import 'dart:async';
 import 'package:compiler/compiler_new.dart';
 
 class BufferedOutputSink implements OutputSink {
   StringBuffer sb = new StringBuffer();
   String text;
 
   void add(String event) {
     sb.write(event);
   }
 
@@ skipping 54 matching lines @@
     return false;
   }
 
   @override
   OutputSink createOutputSink(String name, String extension, OutputType type) {
     Map<String, BufferedOutputSink> sinkMap =
         outputMap.putIfAbsent(type, () => {});
     return sinkMap.putIfAbsent(name, () => new BufferedOutputSink());
   }
 }