Adding client tests (dart-lang#95)

Author: donny-dont

test/client.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2017, 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 'package:http/http.dart';
+
+Client platformClient() => null;
+
+String userAgent() => null;