dart2js: run dartfmt on tests

tests/compiler/dart2js/source_map_validator_test_file.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.
 
 class HelloClass {
   void printHello() {
     () {
       print('Hello World!');
     }();
   }
 }
 
 void main() {
   () {
     HelloClass helloClass = new HelloClass();
     helloClass.printHello();
   }();
-}
+}