flutter create

-platforms

=android,ios

-template

=plugin native_add

cd native_add

Note:
  You can exclude platforms from –platforms that you don’t want
  to build to. However, you need to include the platform of 
  the device you are testing on.

Step 2: Add C/C++ sources

You need to inform the Android build system about
the native code so the code can be compiled
and linked appropriately into the final application.

You can add Android-specific sources
to the android folder and modify CMakeLists.txt
appropriately.
Also, Gradle allows you to point to the ios folder,
if that helps, but it’s not required to use the same
sources for both iOS and Android;