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

sky/sdk/BUILD.gn

 # 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("//mojo/public/dart/rules.gni")
 
 dart_pkg("sdk") {
   sources = [
     "CHANGELOG.md",
     "bin/init.dart",
@@ skipping 75 matching lines @@
     "lib/framework/elements/sky-menu-divider.sky",
     "lib/framework/elements/sky-menu-item.sky",
     "lib/framework/elements/sky-radio.sky",
     "lib/framework/elements/sky-scrollable.sky",
     "lib/framework/elements/sky-toolbar.sky",
     "lib/framework/embedder.dart",
     "lib/framework/fn.dart",
     "lib/framework/fn2.dart",
     "lib/framework/layout.dart",
     "lib/framework/net/fetch.dart",
+    "lib/framework/net/image_cache.dart",
     "lib/framework/node.dart",
     "lib/framework/reflect.dart",
     "lib/framework/rendering/render_block.dart",
     "lib/framework/rendering/render_box.dart",
     "lib/framework/rendering/render_flex.dart",
     "lib/framework/rendering/render_node.dart",
     "lib/framework/rendering/render_paragraph.dart",
     "lib/framework/scheduler.dart",
     "lib/framework/shell.dart",
     "lib/framework/theme/colors.dart",
     "lib/framework/theme/shadows.dart",
     "lib/framework/theme/typography.dart",
     "lib/framework/theme/view_configuration.dart",
     "lib/sky_tool",
     "pubspec.yaml",
   ]
 
   deps = [
     "//sky/services/testing:bindings",
   ]
 
   datadeps = [
     "//sky/engine/bindings",
   ]
 
   sdk_ext_directory = "$root_gen_dir/sky/bindings"
 }