dart2js: Fix hints in code base.

pkg/compiler/lib/src/js/js.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.
 
 library js;
 
 import 'package:js_ast/js_ast.dart';
 export 'package:js_ast/js_ast.dart';
 
-import '../helpers/helpers.dart';
 import '../io/code_output.dart' show CodeBuffer;
 import '../io/source_information.dart' show SourceInformation;
 import '../js_emitter/js_emitter.dart' show USE_NEW_EMITTER;
 import '../dart2jslib.dart' as leg;
 import '../util/util.dart' show NO_LOCATION_SPANNABLE;
 import '../dump_info.dart' show DumpInfoTask;
 
 CodeBuffer prettyPrint(Node node, leg.Compiler compiler,
                        {DumpInfoTask monitor,
                         bool allowVariableMinification: true}) {
@@ skipping 58 matching lines @@
       if (rootNode == node) {
         outBuffer.addSourceLocation(endPosition, null);
         rootNode = null;
       }
     }
     if (monitor != null) {
       monitor.recordAstSize(node, endPosition - startPosition);
     }
   }
 }