Rename .sdkext to _sdkext and fix sky_tool

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/_sdkext",
     "lib/animation/animated_value.dart",
     "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/assets/.gitignore",
     "lib/assets/material-design-icons.sha1",
     "lib/base/debug.dart",
@@ skipping 114 matching lines @@
     ":sky",
   ]
 }
 
 group("sdk") {
   deps = [
     ":sky",
     ":material_design_icons",
   ]
 }