This adds avoid_dynamic_calls to the list of lints, and fixes all instances where it was violated.

Importantly, this lint is NOT turned on for flutter/packages/test, because those changes are happening in another PR: #84478

Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.

update hot reload benchmark to be more representative

This allows the two videos in the demo to play simultaneously
(matching the behavior from before the recent upgrade to the
video player plugin)

This reverts commit 7ba775a9.