Build skyx packages by default

sky/sdk/example/mine_digger/BUILD.gn

Index: sky/sdk/example/mine_digger/BUILD.gn
diff --git a/sky/sdk/example/mine_digger/BUILD.gn b/sky/sdk/example/mine_digger/BUILD.gn
index 3472599a817da312785f278112ea7b46fecc6417..ec5218b15ae529f7bc5bc034a2d4c67a5608686e 100644
--- a/sky/sdk/example/mine_digger/BUILD.gn
+++ b/sky/sdk/example/mine_digger/BUILD.gn
@@ -2,11 +2,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-assert(is_android)
-
 import("//sky/build/sky_app.gni")
 
 sky_app("mine_digger") {
-  apk_name = "MineDigger"
   main_dart = "lib/main.dart"
+
+  if (is_android) {
+    apk_name = "MineDigger"
+  }
 }