dart2js: run dartfmt on tests

tests/compiler/dart2js/mixin_constructor_default_parameter_values_test.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.
 
 // Ensure that the inferrer looks at default values for parameters in
 // synthetic constructors using the correct context. If the constructor call
 // to D without optional parameters is inferred using D's context, the default
 // value `_SECRET` will not be visible and compilation will fail.
 
 import 'package:async_helper/async_helper.dart';
@@ skipping 25 matching lines @@
     C.a(int x, [var b = const _SECRET()]) : this.x = x, this.y = b;
     C.b(int x, {var b : const _SECRET()}) : this.x = x, this.y = b;
     String toString() => "C($x,$y)";
   }
 """
 };
 
 main() {
   asyncTest(() => runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES));
 }
-