Release builds of your app will now be signed automatically.

Note:
  You may need to run flutter clean after changing the gradle file.
  This prevents cached builds from affecting the signing process.

For more information on signing your app, see
Sign your app on developer.android.com.

Shrinking your code with R8

R8 is the new code shrinker from Google, and it’s enabled by default
when you build a release APK or AAB. To disable R8, pass the --no-shrink
flag to flutter build apk or flutter build appbundle.

Note:
  Obfuscation and minification can considerably extend compile time
  of the Android application.

Enabling multidex support