Implementation of null-aware operators.

tests/language/conditional_access_helper.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.
 
 // Library used by conditional_property_assignment_test.dart,
 // conditional_property_access_test.dart, and
 // conditional_method_invocation_test.dart, all of which import it using the
 // prefix "h.".
 
 library lib;
 
 var topLevelVar;
 
+void topLevelFunction() {}
+
 class C {
   static var staticField;
+  static void staticMethod() {}
 }