Add operators test to source_mapping_test.

tests/compiler/dart2js/sourcemaps/invokes_test_file.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.
 
 // Test file for testing source mappings of invocations.
 
 var counter = 0;
-var bucket;
 
 main(args) {
   counter++;
   invokes(args);
   return counter;
 }
 
 invokes(parameter) {
   counter++;
   toplevelFunction();
@@ skipping 100 matching lines @@
     super.superMethod();
     super.superField();
     super.superGetter();
 
     C();
     dynamic();
     F();
     T();
   }
 }