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

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 05a3340e10d0d5fa830bbdbfa24d82785d61bd4e..94890e3563d699bda7e22f68bc5018275b108bd6 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -793,7 +793,7 @@ class Firefox extends Browser {
       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");