Suppress get_drt/dartium download if an executable is provided via flag

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 35 matching lines @@
 
   void _onUpdatedHandler(int exit_code) {
     print('$name updated ($exit_code)');
     for (var callback in onUpdated ) callback();
   }
 }
 
 _DartiumUpdater _dumpRenderTreeUpdater;
 _DartiumUpdater _dartiumUpdater;
 
-_DartiumUpdater runtimeUpdater(String runtime) {
-  if (runtime == 'drt') {
+_DartiumUpdater runtimeUpdater(Map configuration) {
+  String runtime = configuration['runtime'];
+  if (runtime == 'drt' && configuration['drt'] == '') {
+    // Download the default DumpRenderTree from Google Storage.
     if (_dumpRenderTreeUpdater === null) {
       _dumpRenderTreeUpdater = new _DartiumUpdater('DumpRenderTree',
                                                    'get_drt.py');
     }
     return _dumpRenderTreeUpdater;
-  } else if (runtime == 'dartium') {
+  } else if (runtime == 'dartium' && configuration['dartium'] == '') {
+    // Download the default Dartium from Google Storage.
     if (_dartiumUpdater === null) {
       _dartiumUpdater = new _DartiumUpdater('Dartium Chrome', 'get_drt.py',
                                             '--dartium');
     }
     return _dartiumUpdater;
   } else {
     return null;
   }
 }