[platform_channels] adds Platform Image demo (#475)

platform_channels/test/src/platform_image_demo_test.dart

@@ -0,0 +1,38 @@
+// 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 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:platform_channels/src/platform_image_demo.dart';
+
+void main() {
+  group('Platform Image Demo tests', () {
+    setUpAll(() {
+      // Register a mock for MessageHandler.
+      BasicMessageChannel<dynamic>('platformImageDemo', StandardMessageCodec())
+          .setMockMessageHandler((dynamic message) async {
+        var byteData = await rootBundle.load('assets/eat_new_orleans.jpg');
+        return byteData.buffer.asUint8List();
+      });
+    });
+
+    testWidgets('Platform Image test', (tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: PlatformImageDemo(),
+      ));
+
+      // Initially a PlaceHolder is displayed when imageData is null.
+      expect(find.byType(Placeholder), findsOneWidget);
+      expect(find.byType(Image), findsNothing);
+
+      // Tap on RaisedButton to get Image.
+      await tester.tap(find.byType(RaisedButton));
+      await tester.pumpAndSettle();
+
+      expect(find.byType(Placeholder), findsNothing);
+      expect(find.byType(Image), findsOneWidget);
+    });
+  });
+}