dart2js: run dartfmt on tests

tests/compiler/dart2js_extra/mirrors_used_metatargets_test.dart

 // Copyright (c) 2013, 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.
 
 @MirrorsUsed(metaTargets: 'X')
 import 'dart:mirrors';
 
 const x = const X();
+
 class X {
   const X();
 }
 
 @x
 class Y {
   foo() => 42;
 }
 
 class Z {
@@ skipping 11 matching lines @@
   if (reflect(z).invoke(#bar, []).reflectee != 87) throw 'Wrong Z.bar';
 
   bool caught = false;
   try {
     reflect(z).invoke(#foo, []);
   } on UnsupportedError catch (e) {
     caught = true;
   }
   if (!caught) throw 'Wrong Z.foo';
 }