Fixing sprites' image map regression. (#4628)

* Fixing sprites' image map regression.
* Smoke tests for ImageMap

packages/flutter_sprites/lib/src/image_map.dart

@@ -21,7 +21,7 @@ class ImageMap {
   }
 
   Future<ui.Image> _loadImage(String url) async {
-    ImageStream stream = new NetworkImage(url).resolve(ImageConfiguration.empty);
+    ImageStream stream = new AssetImage(url, bundle: _bundle).resolve(ImageConfiguration.empty);
     Completer<ui.Image> completer = new Completer<ui.Image>();
     void listener(ImageInfo frame) {
       final ui.Image image = frame.image;

packages/flutter_sprites/test/image_map_test.dart

+// Copyright 2015 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' show BASE64;
+import 'dart:typed_data';
+import 'dart:ui' as ui show Image;
+
+import 'package:flutter/services.dart';
+import 'package:flutter_sprites/flutter_sprites.dart';
+import 'package:mojo/core.dart' as mojo;
+import 'package:test/test.dart';
+
+const String kTestManifest = '''
+{
+  "assets/image1.png" : [],
+  "assets/image2.png" : [],
+  "assets/image3.png" : []
+}
+''';
+
+// Base64 encoding of a 1x1 pixel png image.
+const String kTestImageBase64 =
+    'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEUAAACnej3aAAAAAXRSTlMAQObYZgAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=';
+
+class TestAssetBundle extends CachingAssetBundle {
+  @override
+  Future<mojo.MojoDataPipeConsumer> load(String key) {
+    mojo.MojoDataPipe dataPipe = new mojo.MojoDataPipe();
+    final Uint8List bytes =
+        new Uint8List.fromList(BASE64.decode(kTestImageBase64));
+    int numBytesWritten = dataPipe.producer.write(bytes.buffer.asByteData());
+    expect(numBytesWritten, equals(bytes.lengthInBytes));
+    dataPipe.producer.handle.close();
+    return new Future<mojo.MojoDataPipeConsumer>.value(dataPipe.consumer);
+  }
+
+  @override
+  Future<String> loadString(String key, {bool cache: true}) {
+    if (key == 'AssetManifest.json')
+      return new Future<String>.value(kTestManifest);
+    return null;
+  }
+
+  @override
+  String toString() => '$runtimeType@$hashCode()';
+}
+
+final TestAssetBundle _bundle = new TestAssetBundle();
+
+void main() {
+  test('ImageMap Smoke Test', () async {
+    ImageMap imageMap = new ImageMap(_bundle);
+    final List<String> urls = <String>[
+      'assets/image1.png',
+      'assets/image2.png',
+      'assets/image3.png',
+    ];
+
+    urls.forEach((String url) {
+      expect(imageMap.getImage(url), isNull);
+    });
+
+    List<ui.Image> loadedImages = await imageMap.load(urls);
+    expect(loadedImages.length, equals(urls.length));
+
+    urls.forEach((String url) {
+      expect(imageMap.getImage(url), isNotNull);
+    });
+  });
+}