Add browser path options to testing scripts.

tools/testing/dart/launch_browser.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.
 
 /**
  * Simple command line interface to launching browsers.
  * Uses the browser_controller framework.
  * The usage is:
  *   DARTBIN launch_browser.dart BROWSER_NAME URL
  * DARTBIN should be the checked in stable binary.
  */
 
 import "dart:io";
 import "browser_controller.dart";
 
 void printHelp() {
   print("Usage pattern:");
   print("launch_browser.dart browser url");
   print("Supported browsers: ${Browser.SUPPORTED_BROWSERS}");
 }
 
 void main() {
   var args = new Options().arguments;
   if (args.length != 2) {
     print("Wrong number of arguments, please pass in exactly two arguments");
     printHelp();
     return;
   }
+  var name = args[0];
 
-  if (!Browser.supportedBrowser(args[0])) {
+  if (!Browser.supportedBrowser(name)) {
     print("Specified browser not supported");
     printHelp();
     return;
   }
 
-  var browser = new Browser.byName(args[0]);
+  var executable = Locations.getBrowserExecutable(name, {});
+  var browser = new Browser.byName(name, executable);
   browser.start(args[1]);
-
 }