Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/dart2js.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.
 
 library dart2js.cmdline;
 
-import 'dart:async' show EventSink, Future;
+import 'dart:async' show Future;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io' show exit, File, FileMode, Platform, stdin, stderr;
 
 import 'package:package_config/discovery.dart' show findPackages;
 
 import '../compiler_new.dart' as api;
 import 'commandline_options.dart';
 import 'common/names.dart' show Uris;
 import 'filenames.dart';
 import 'io/source_file.dart';
@@ skipping 1019 matching lines @@
   @override
   void add(String event) {
     sb.write(event);
   }
 
   @override
   void close() {
     // Do nothing.
   }
 }