Hoist knowledge of image_cache into widgets/basic.dart

sky/sdk/lib/mojo/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:async';
+import 'dart:collection';
 import 'dart:sky' as sky;
-import 'dart:collection';
 
 import 'package:mojom/mojo/url_response.mojom.dart';
 
 import 'fetch.dart';
 
-final HashMap<String, List<sky.ImageDecoderCallback>> _pendingRequests =
-  new HashMap<String, List<sky.ImageDecoderCallback>>();
+final HashMap<String, Future<sky.Image>> _cache =
+    new HashMap<String, Future<sky.Image>>();
 
-final HashMap<String, sky.Image> _completedRequests =
-  new HashMap<String, sky.Image>();
-
-void _loadComplete(String url, sky.Image image) {
-  _completedRequests[url] = image;
-  _pendingRequests[url].forEach((c) => c(image));
-  _pendingRequests.remove(url);
-}
-
-void _load(String url, sky.ImageDecoderCallback 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.ImageDecoderCallback>();
-  }).add(callback);
-  if (newRequest) {
+Future<sky.Image> load(String url) {
+  return _cache.putIfAbsent(url, () {
+    Completer<sky.Image> completer = new Completer<sky.Image>();
     fetchUrl(url).then((UrlResponse response) {
       if (response.statusCode >= 400) {
-        _loadComplete(url, null);
-        return;
+        completer.complete(null);
+      } else {
+        new sky.ImageDecoder(response.body.handle.h, completer.complete);
       }
-      new sky.ImageDecoder(response.body.handle.h, (image) {
-        _loadComplete(url, image);
-      });
     });
-  }
+    return completer.future;
+  });
 }
-
-Future<sky.Image> load(String url) {
-  Completer<sky.Image> completer = new Completer<sky.Image>();
-  _load(url, completer.complete);
-  return completer.future;
-}