Make compatible with new RenderObject.dispose API

[dnfield]

After https://github.com/flutter/flutter/commit/c1fe2bd1e0fef2d9f54d51c59f58cb32c33a0726, RenderObject.dispose expects overrides to call super.

The framework now calls this method automatically at the right times, so the manual calls can be removed.

[dnfield]

This should probbly get a version constraint added to pubspec.yaml, but the changes haven't been published to flutter dev yet - they will be soon.

[luigi-rosso]

Is the change backwards compatible or do we need to wait for this to land in stable?

[dnfield]

This is not backwards compatible. Would you be open to publishing it with a version constraint? The version it first appears in is 2.3.0-24.0.pre

[dnfield]

(this is now the API used on stable)