Add helpers library to dart2js.

sdk/lib/_internal/compiler/implementation/source_map_builder.dart

Index: sdk/lib/_internal/compiler/implementation/source_map_builder.dart
diff --git a/sdk/lib/_internal/compiler/implementation/source_map_builder.dart b/sdk/lib/_internal/compiler/implementation/source_map_builder.dart
index 1f5e90843bf1a7310d46d28bd6c5a12c7a265350..3910615899285e634f082a70081b575ca00e45fe 100644
--- a/sdk/lib/_internal/compiler/implementation/source_map_builder.dart
+++ b/sdk/lib/_internal/compiler/implementation/source_map_builder.dart
@@ -4,7 +4,6 @@
 
 library source_map_builder;
 
-
 import 'util/util.dart';
 import 'scanner/scannerlib.dart' show Token;
 import 'source_file.dart';
@@ -215,12 +214,3 @@ class TokenSourceFileLocation extends SourceFileLocation {
     return null;
   }
 }
-
-class OffsetSourceFileLocation extends SourceFileLocation {
-  final int offset;
-  final String sourceName;
-  OffsetSourceFileLocation(SourceFile sourceFile, this.offset,
-      [this.sourceName]) : super(sourceFile);
-
-  String getSourceName() => sourceName;
-}