`tns migrate` clears the platforms and next build fails if a custom android runtime was used

If tns migrate was ran on an app with a custom android runtime (tns platform add android --frameworkPath /test/runtime.tgz), next build would fail. This is because the platforms/android folder is deleted and the version of the tns-android package was previously set in the package.json to the literal next version of the runtime. This runtime, of course, couldn't be found in npm and thus the build fails. Couldn't runtimes added with --frameworkPath be handled differently and in some way marked as such in the package.json? Also, couldn't a simple gradlew clean do the dirty work instead of deleting the whole platforms folder?

まず tns migrate のエントリポイントと --frameworkPath の処理を追跡し、次に package.json が tns-android のバージョンをどのように記録しているか、また platforms/android がどのように削除されるかを調べます。その動作を、言及されている gradlew clean オプションと比較します。カスタムランタイムを使用するアプリを移行した際に、使用可能なランタイム参照が保持され、次のビルドが成功すれば完了です。