Create a valid no-op dylib for the iOS simulator. (#2718)

sky/build/sdk_xcode_harness/Tools/common/BuildFlutterApp

@@ -64,7 +64,7 @@ BuildApp() {
       exit -1
     fi
   else
-    RunCommand touch ${derived_dir}/app.so
+    RunCommand eval "$(echo \"static const int Moo = 88;\" | xcrun clang -x c --shared -o ${derived_dir}/app.so -)"
   fi
 
   local precompilation_flag=""