Implement a simple image cache for Sky in Dart (no eviction)

sky/sdk/lib/framework/net/image_cache.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:sky' as sky;
+import 'dart:collection';
+
+final HashMap<String, List<sky.ImageLoaderCallback>> _pendingRequests =
+  new HashMap<String, List<sky.ImageLoaderCallback>>();
+
+final HashMap<String, sky.Image> _completedRequests =
+  new HashMap<String, sky.Image>();
+
+void load(String url, sky.ImageLoaderCallback callback) {
+  sky.Image result = _completedRequests[url];
+  if (result != null) {
+    callback(_completedRequests[url]);
+    return;
+  }
+
+  bool newRequest = false;
+  _pendingRequests.putIfAbsent(url, () {
+    newRequest = true;
+    return new List<sky.ImageLoaderCallback>();
+  }).add(callback);
+  if (newRequest) {
+    new sky.ImageLoader(url, (image) {
+      _completedRequests[url] = image;
+      _pendingRequests[url].forEach((c) => c(image));
+      _pendingRequests.remove(url);
+    });
+  }
+}