Remove deprecated sourceMapping syntax.

tests/compiler/dart2js/source_map_validator_helper.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:io';
 import 'dart:convert';
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 import 'package:expect/expect.dart';
@@ skipping 158 matching lines @@
       (entry.sourceLine == otherEntry.sourceLine) &&
       (entry.sourceColumn == otherEntry.sourceColumn) &&
       (entry.sourceNameId == otherEntry.sourceNameId);
 }
 
 Uri getMapUri(Uri targetUri) {
   print('Accessing $targetUri');
   File targetFile = new File.fromUri(targetUri);
   Expect.isTrue(targetFile.existsSync());
   List<String> target = targetFile.readAsStringSync().split('\n');
-  String mapReference = target[target.length - 3]; // #sourceMappingURL=<url>
+  String mapReference = target[target.length - 2]; // #sourceMappingURL=<url>
   Expect.isTrue(mapReference.startsWith('//# sourceMappingURL='));
   String mapName = mapReference.substring(mapReference.indexOf('=') + 1);
   return targetUri.resolve(mapName);
 }
 
 SingleMapping getSourceMap(Uri mapUri) {
   print('Accessing $mapUri');
   File mapFile = new File.fromUri(mapUri);
   Expect.isTrue(mapFile.existsSync());
   return new SingleMapping.fromJson(
@@ skipping 41 matching lines @@
   final Position end;
 
   Interval(this.begin, this.end);
 
   bool contains(Position other) {
     return begin <= other && other <= end;
   }
 
   String toString() => '$begin-$end';
 }