Move VariableHeightScrollable to sdk/lib/widgets

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",
@@ skipping 68 matching lines @@
     "lib/widgets/scaffold.dart",
     "lib/widgets/scrollable.dart",
     "lib/widgets/scrollable_list.dart",
     "lib/widgets/snack_bar.dart",
     "lib/widgets/switch.dart",
     "lib/widgets/tabs.dart",
     "lib/widgets/task_description.dart",
     "lib/widgets/theme.dart",
     "lib/widgets/toggleable.dart",
     "lib/widgets/tool_bar.dart",
+    "lib/widgets/variable_height_scrollable.dart",
     "lib/widgets/widget.dart",
     "pubspec.yaml",
   ]
 
   # List of mojom targets that the sky pkg exports
   deps = [
     "//sky/services/engine:interfaces",
     "//sky/services/activity:interfaces",
   ]
 
@@ skipping 44 matching lines @@
     ":sky",
   ]
 }
 
 group("sdk") {
   deps = [
     ":sky",
     ":material_design_icons",
   ]
 }