Fix some type warnings in the test scripts, that show up when using the editor.

tools/testing/dart/drt_updater.dart

 // Copyright (c) 2012, 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("drt_updater");
 
 #import("dart:io");
 #import("dart:builtin");
 
 class _DartiumUpdater {
@@ skipping 16 matching lines @@
       onUpdated = [() {updated = true;} ];
       _updatingProcess = new Process.start('python', _getUpdateCommand);
       _updatingProcess.onExit = _onUpdatedHandler;
       _updatingProcess.onError = (error) {
         print("Error starting $script process: $error");
         _onUpdatedHandler(-1);  // Continue anyway.
       };
     }
   }
 
-  String get _getUpdateCommand() {
+  List<String> get _getUpdateCommand() {
     String scriptPath = new Options().script.replaceAll('\\', '/');
     String toolsDir = scriptPath.substring(0, scriptPath.lastIndexOf('/'));
     List<String> command = ['$toolsDir/$script'];
     if (null !== option) {
       command.add(option);
     }
     return command;
   }
 
   void _onUpdatedHandler(int exit_code) {
@@ skipping 15 matching lines @@
   } else if (runtime == 'dartium') {
     if (_dartiumUpdater === null) {
       _dartiumUpdater = new _DartiumUpdater('Dartium Chrome', 'get_drt.py',
                                             '--dartium');
     }
     return _dartiumUpdater;
   } else {
     return null;
   }
 }