Add helpers library to dart2js.

sdk/lib/_internal/compiler/implementation/source_map_builder.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.
 
 library source_map_builder;
 
-
 import 'util/util.dart';
 import 'scanner/scannerlib.dart' show Token;
 import 'source_file.dart';
 import 'util/uri_extras.dart' show relativize;
 
 class SourceMapBuilder {
   static const int VLQ_BASE_SHIFT = 5;
   static const int VLQ_BASE_MASK = (1 << 5) - 1;
   static const int VLQ_CONTINUATION_BIT = 1 << 5;
   static const int VLQ_CONTINUATION_MASK = 1 << 5;
@@ skipping 190 matching lines @@
   TokenSourceFileLocation(SourceFile sourceFile, this.token)
     : super(sourceFile);
 
   int get offset => token.charOffset;
 
   String getSourceName() {
     if (token.isIdentifier()) return token.value;
     return null;
   }
 }
-
-class OffsetSourceFileLocation extends SourceFileLocation {
-  final int offset;
-  final String sourceName;
-  OffsetSourceFileLocation(SourceFile sourceFile, this.offset,
-      [this.sourceName]) : super(sourceFile);
-
-  String getSourceName() => sourceName;
-}