Add helpers library to dart2js.

sdk/lib/_internal/compiler/implementation/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 Future, EventSink;
 import 'dart:io'
     show exit, File, FileMode, Platform, RandomAccessFile, FileSystemException;
 import 'dart:math' as math;
 
 import '../compiler.dart' as api;
 import 'source_file.dart';
 import 'source_file_provider.dart';
 import 'filenames.dart';
 import 'util/uri_extras.dart';
-import 'util/util.dart';
+import 'util/util.dart' show stackTraceFilePrefix;
 import '../../libraries.dart';
 
 const String LIBRARY_ROOT = '../../../../..';
 const String OUTPUT_LANGUAGE_DART = 'Dart';
 
 /**
  * A string to identify the revision or build.
  *
  * This ID is displayed if the compiler crashes and in verbose mode, and is
  * an aid in reproducing bug reports.
@@ skipping 627 matching lines @@
     }
   }
 
   try {
     return compilerMain(arguments).catchError(onError);
   } catch (exception, trace) {
     onError(exception, trace);
     return new Future.value();
   }
 }