dart2js: run dartfmt on tests

tests/compiler/dart2js/mixin_language_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.
 
 // Test that dart2js produces the expected static type warnings for these
 // language tests. This ensures that the analyzer and dart2js agrees on the
 // tests.
 
 import 'warnings_checker.dart';
 
 /// Map from test files to a map of their expected status. If the status map is
 /// `null` no warnings must be missing or unexpected, otherwise the status map
 /// can contain a list of line numbers for keys 'missing' and 'unexpected' for
 /// the warnings of each category.
 const Map<String, dynamic> TESTS = const {
-    'language/typevariable_substitution2_test.dart': null,
+  'language/typevariable_substitution2_test.dart': null,
 };
 
 void main(List<String> arguments) {
   checkWarnings(TESTS, arguments);
 }