Do some housekeeping in tools/testing/dart

tools/testing/dart/test_options.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.
 
 library test_options_parser;
 
 import "dart:io";
 import "drt_updater.dart";
 import "test_suite.dart";
 import "path.dart";
@@ skipping 997 matching lines @@
    * Find the test option specification for a given option key.
    */
   _TestOptionSpecification _getSpecification(String name) {
     for (var option in _options) {
       if (option.keys.contains(name)) {
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
+    return null; // Unreachable.
   }
 
   List<_TestOptionSpecification> _options;
 }