Generate Sky's _sdkext file using the build system

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("sky") {
   sources = [
     "CHANGELOG.md",
     "bin/init.dart",
-    "lib/_sdkext",
     "lib/animation/animation_performance.dart",
     "lib/animation/curves.dart",
     "lib/animation/fling_curve.dart",
     "lib/animation/generators.dart",
     "lib/animation/mechanics.dart",
     "lib/animation/scroll_behavior.dart",
     "lib/animation/timeline.dart",
     "lib/assets/.gitignore",
     "lib/assets/material-design-icons.sha1",
     "lib/base/debug.dart",
@@ skipping 84 matching lines @@
     "//sky/services/testing:interfaces",
     "//sky/services/media:interfaces",
     "//third_party/dart-pkg",
   ]
 
   sdk_ext_directory = "$root_gen_dir/sky/bindings"
   sdk_ext_files = [
     "//sky/engine/bindings/sky_internals.dart",
     "//sky/engine/bindings/builtin_natives.dart",
   ]
+  sdk_ext_mappings = [
+    "dart:sky,dart_sky.dart",
+    "dart:sky.internals,sky_internals.dart",
+    "dart:sky_builtin_natvies,builtin_natives.dart",
+  ]
 }
 
 action("material_design_icons") {
   input_dir = "lib/assets/material-design-icons"
   output_dir = "$root_gen_dir/dart-pkg/sky/lib/assets"
   stamp = "$target_gen_dir/material_design_icons_linked"
 
   sources = [
     "lib/assets/material-design-icons.sha1",
   ]
@@ skipping 14 matching lines @@
     ":sky",
   ]
 }
 
 group("sdk") {
   deps = [
     ":sky",
     ":material_design_icons",
   ]
 }