Do some housekeeping in tools/testing/dart

tools/testing/dart/test_progress.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_progress;
 
 import "dart:async";
 import "dart:io";
 import "dart:io" as io;
 import "dart:convert" show JSON;
@@ skipping 676 matching lines @@
     case 'line':
       return new LineProgressIndicator();
     case 'verbose':
       return new VerboseProgressIndicator(startTime);
     case 'status':
       return new ProgressIndicator(startTime);
     case 'buildbot':
       return new BuildbotProgressIndicator(startTime);
     default:
       assert(false);
-      break;
+      return null;
   }
 }