Add package:sky/mojo/asset_bundle.dart to wrap mojo:asset_bundle

This Dart library exposes a more idiomatic interface to the asset_bundle
service and provides integration with dart:sky.

R=viktorl@google.com

Review URL: https://codereview.chromium.org/1200463002.

sdk/BUILD.gn

@@ -71,6 +71,7 @@ dart_pkg("sdk") {
     "lib/framework/theme/typography.dart",
     "lib/framework/theme/view_configuration.dart",
     "lib/internals.dart",
+    "lib/mojo/asset_bundle.dart",
     "lib/mojo/embedder.dart",
     "lib/mojo/net/fetch.dart",
     "lib/mojo/net/image_cache.dart",

sdk/lib/mojo/asset_bundle.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:sky' as sky;
+import 'dart:typed_data';
+
+import 'package:mojo/core.dart' as core;
+import 'package:mojom/mojo/asset_bundle/asset_bundle.mojom.dart';
+
+import 'shell.dart' as shell;
+import 'net/fetch.dart';
+
+Future<sky.Image> _decodeImage(core.MojoDataPipeConsumer assetData) {
+  Completer<sky.Image> completer = new Completer();
+  new sky.ImageDecoder(assetData.handle.h, completer.complete);
+  return completer.future;
+}
+
+class AssetBundle {
+  AssetBundle(AssetBundleProxy this._bundle);
+
+  void close() {
+    _bundle.close();
+    _bundle = null;
+  }
+
+  Future<sky.Image> fetchImage(String path) async {
+    core.MojoDataPipeConsumer assetData =
+        (await _bundle.ptr.getAsStream(path)).assetData;
+    return await _decodeImage(assetData);
+  }
+
+  AssetBundleProxy _bundle;
+}
+
+Future<AssetBundle> fetchAssetBundle(String url) async {
+  assert(false); // mojo:asset_bundle doesn't exist yet, see https://codereview.chromium.org/1176153007
+  core.MojoDataPipeConsumer bundleData = (await fetchUrl(url)).body;
+
+  AssetUnpackerProxy unpacker = new AssetUnpackerProxy.unbound();
+  shell.requestService("mojo:asset_bundle", unpacker);
+  AssetBundleProxy bundle = new AssetBundleProxy.unbound();
+  unpacker.ptr.unpackZipStream(bundleData, bundle);
+  unpacker.close();
+
+  return new AssetBundle(bundle);
+}