Revert "Remove support for "packages" URL from testing server."

tools/testing/dart/browser_controller.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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:core';
 import 'dart:io';
 import 'dart:math';
 
@@ skipping 775 matching lines @@
     randomFile.close();
   }
 
   Future<bool> start(String url) {
     _logEvent("Starting firefox browser on: $url");
     // Get the version and log that.
     return Process.run(_binary, ["--version"]).then((var versionResult) {
       if (versionResult.exitCode != 0) {
         _logEvent("Failed to firefox get version");
         _logEvent("Make sure $_binary is a valid program for running firefox");
-        return new Future<bool>.value(false);
+        return new Future.value(false);
       }
       version = versionResult.stdout as String;
       _logEvent("Got version: $version");
 
       return Directory.systemTemp.createTemp().then((userDir) {
         _createPreferenceFile(userDir.path);
         _cleanup = () {
           userDir.deleteSync(recursive: true);
         };
         var args = [
@@ skipping 1111 matching lines @@
           'exit code: ${result.exitCode}\n'
           'stdout: ${result.stdout}\n'
           'stderr: ${result.stderr}');
     } else {
       print('[$message] Successfully uploaded screenshot to $storageUrl');
     }
     new File(screenshotFile).deleteSync();
   }
   print('--------------------------------------------------------------------');
 }