dart2js: run dartfmt on tests

tests/compiler/dart2js_extra/source_mapping_crash_test.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_mapping_crash_test;
 
 part 'source_mapping_crash_source.dart';
 
 class Sub extends Super {
   Sub(var x) : super(x.y);
 }
 
-class X { var y; }
+class X {
+  var y;
+}
 
 main() {
   new Sub(new X());
 }