dart2js: Fix hints in code base.

pkg/compiler/lib/src/patch_parser.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This library contains the infrastructure to parse and integrate patch files.
  *
  * Three types of elements can be patched: [LibraryElement], [ClassElement],
  * [FunctionElement]. Patches are introduced in patch libraries which are loaded
  * together with the corresponding origin library. Which libraries that are
@@ skipping 111 matching lines @@
          CompilerTask,
          DiagnosticListener,
          MessageKind,
          Script;
 import 'elements/elements.dart';
 import 'elements/modelx.dart'
     show LibraryElementX,
          MetadataAnnotationX,
          ClassElementX,
          BaseFunctionElementX;
-import 'helpers/helpers.dart';  // Included for debug helpers.
 import 'library_loader.dart' show LibraryLoader;
 import 'scanner/scannerlib.dart';  // Scanner, Parsers, Listeners
 import 'util/util.dart';
 
 class PatchParserTask extends CompilerTask {
   final String name = "Patching Parser";
 
   PatchParserTask(Compiler compiler): super(compiler);
 
   /**
@@ skipping 422 matching lines @@
 
 class PatchVersion {
   final String tag;
 
   const PatchVersion(this.tag);
 
   bool isActive(String patchTag) => tag == null || tag == patchTag;
 
   String toString() => 'PatchVersion($tag)';
 }