Add skyx packages for all the Sky demos (except game)

sky/sdk/example/widgets/BUILD.gn

Index: sky/sdk/example/widgets/BUILD.gn
diff --git a/apps/BUILD.gn b/sky/sdk/example/widgets/BUILD.gn
similarity index 53%
copy from apps/BUILD.gn
copy to sky/sdk/example/widgets/BUILD.gn
index 59db4eefb1910915c21789af1cd6f423cf602237..299ad5b520476de60a49a853a3aa66f81109dab9 100644
--- a/apps/BUILD.gn
+++ b/sky/sdk/example/widgets/BUILD.gn
@@ -2,13 +2,21 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-group("apps") {
+import("//sky/build/skyx.gni")
+
+skyx("sector") {
+  main_dart = "sector.dart"
+}
+
+skyx("piano") {
+  main_dart = "piano.dart"
+}
+
+group("widgets") {
   testonly = true
 
   deps = [
-    "//apps/moterm",
-    "//apps/moterm:apptests",
-    "//apps/moterm:gl_helper_test_app",
-    "//apps/moterm:moterm_example_app",
+    ":sector",
+    ":piano",
   ]
 }