dart2js: run dartfmt on tests

tests/compiler/dart2js_extra/mirrors_used_native_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(targets: 'List')
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 main() {
   Expect.equals(3, reflect([1, 2, 3]).getField(#length).reflectee);
   Expect.throws(() => reflect({"hest": 42}).getField(#length),
-                (e) => e is UnsupportedError);
+      (e) => e is UnsupportedError);
 }