[platform_channels] adds Platform Image demo (#475)

platform_channels/lib/src/image_basic_message_channel.dart

@@ -0,0 +1,28 @@
+// Copyright 2020 The Flutter team. 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:typed_data';
+
+import 'package:flutter/services.dart';
+
+/// This class manages a [BasicMessageChannel] that can return an image loaded
+/// from a native asset. The [BasicMessageChannel] uses [StandardMessageCodec]
+/// since it supports [Uint8List], which is used to transport the image data.
+class PlatformImageFetcher {
+  static final _basicMessageChannel = const BasicMessageChannel<dynamic>(
+      'platformImageDemo', StandardMessageCodec());
+
+  /// Method responsible for providing the platform image.
+  static Future<Uint8List> getImage() async {
+    final reply = await _basicMessageChannel.send('getImage') as Uint8List;
+    if (reply == null) {
+      throw PlatformException(
+        code: 'Error',
+        message: 'Failed to load Platform Image',
+        details: null,
+      );
+    }
+    return reply;
+  }
+}