Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/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;
@@ skipping 247 matching lines @@
 
   TokenSourceFileLocation(SourceFile sourceFile, this.token, this.name)
     : super(sourceFile);
 
   int get offset => token.charOffset;
 
   String getSourceName() {
     return name;
   }
 }