Tighten types in test.dart even more.

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.
 
 // TODO(antonm): rename to something like test_runner_updater.
 
 library drt_updater;
 
 import "dart:async";
 import "dart:io";
@@ skipping 53 matching lines @@
       exit(1);
     }
     for (var callback in onUpdated) callback();
   }
 }
 
 _DartiumUpdater _contentShellUpdater;
 _DartiumUpdater _dartiumUpdater;
 
 _DartiumUpdater runtimeUpdater(Map configuration) {
-  String runtime = configuration['runtime'];
+  var runtime = configuration['runtime'] as String;
   if (runtime == 'drt' && configuration['drt'] == '') {
     // Download the default content shell from Google Storage.
     if (_contentShellUpdater == null) {
       _contentShellUpdater =
           new _DartiumUpdater('Content Shell', 'tools/get_archive.py', 'drt');
     }
     return _contentShellUpdater;
   } else if (runtime == 'dartium' && configuration['dartium'] == '') {
     // Download the default Dartium from Google Storage.
     if (_dartiumUpdater == null) {
       _dartiumUpdater = new _DartiumUpdater(
           'Dartium Chrome', 'tools/get_archive.py', 'dartium');
     }
     return _dartiumUpdater;
   } else {
     return null;
   }
 }