dart2js: run dartfmt on tests

tests/compiler/dart2js/sourcemaps/source_mapping_operators_test.dart

 // Copyright (c) 2015, 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.
 
 import 'source_mapping_tester.dart';
 import 'sourcemap_helper.dart';
 
 void main() {
   test(['operators'], whiteListFunction: (String config, String file) {
     bool allowGtOptimization(CodePoint codePoint) {
       // Allow missing code points for bailout optimization.
       return codePoint.jsCode.contains(r'.$gt()'); // # Issue 25304
     }
 
     return allowGtOptimization;
   });
-}
+}