Add license header

packages/url_launcher/url_launcher_platform_interface/lib/method_channel_url_launcher.dart

@@ -1,3 +1,7 @@
+// Copyright 2017 The Chromium Authors. 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 'package:flutter/services.dart';