Add Cards and Fitness demos, remove sector.

sky/sdk/example/widgets/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("//sky/build/skyx.gni")
 
 skyx("sector") {
   main_dart = "sector.dart"
 }
 
 skyx("piano") {
   main_dart = "piano.dart"
 }
 
+skyx("cards") {
+  main_dart = "card_collection.dart"
+}
+
 group("widgets") {
   testonly = true
 
   deps = [
+    ":cards",
+    ":piano",
     ":sector",
-    ":piano",
   ]
 }