This PR adds a new constructor to the RefreshIndicator's class, which is `noSpinner`.
The purpose of this new constructor is to create a RefreshIndicator that doesn't show a spinner when the user arms it by pulling.
The work is based on a partial that is here: https://github.com/flutter/flutter/pull/133507
I addressed the following issues reported in the PR above:
- in the example for `noSpinner`, arming the RefreshIndicator now shows a CircularProgressIndicator, instead of just printing text to the console;
- added a test for the new example;
- added a doc comment on the new constructor;
Fixes https://github.com/flutter/flutter/issues/132775
This PR changes the example app into a custom scrollview with three slivers. The middle sliver has a nested scrollview of height 300 and only this nested sliver can trigger the refresh indicator.
Fixes https://github.com/flutter/flutter/issues/116237.
