Add an ergonomic wrapper for platform messages (#6281)

This wrapper will eventually replace HostMessages.

packages/flutter/lib/services.dart

@@ -27,6 +27,7 @@ export 'src/services/image_resolution.dart';
 export 'src/services/image_stream.dart';
 export 'src/services/keyboard.dart';
 export 'src/services/path_provider.dart';
+export 'src/services/platform_messages.dart';
 export 'src/services/shell.dart';
 export 'src/services/system_chrome.dart';
 export 'src/services/system_sound.dart';

packages/flutter/lib/src/services/platform_messages.dart

+// Copyright 2016 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 'dart:convert';
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+
+String _decodeUTF8(ByteData data) {
+  return data != null ? UTF8.decoder.convert(data.buffer.asUint8List()) : null;
+}
+
+dynamic _decodeJSON(String message) {
+  return message != null ? JSON.decode(message) : null;
+}
+
+void _sendString(String name, String message, void callback(String reply)) {
+  Uint8List encoded = UTF8.encoder.convert(message);
+  ui.window.sendPlatformMesssage(name, encoded.buffer.asByteData(), (ByteData reply) {
+    callback(_decodeUTF8(reply));
+  });
+}
+
+/// Sends messages to the hosting application.
+class PlatformMessages {
+  /// Send a string message to the host application.
+  static Future<String> sendString(String name, String message) {
+    Completer<String> completer = new Completer<String>();
+    _sendString(name, message, (String reply) {
+      completer.complete(reply);
+    });
+    return completer.future;
+  }
+
+  /// Sends a JSON-encoded message to the host application and JSON-decodes the response.
+  static Future<dynamic> sendJSON(String name, dynamic json) {
+    Completer<dynamic> completer = new Completer<dynamic>();
+    _sendString(name, JSON.encode(json), (String reply) {
+      completer.complete(_decodeJSON(reply));
+    });
+    return completer.future;
+  }
+}