Also, refactor internal code to do the same.

See https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#avoid-double-negatives-in-apis

This reverts commit 0f8148ec.

* Migrate to ChannelBuffers.push

* Move TestDefaultBinaryMessenger to its own file

* Apply deprecations and fix usages

This reverts commit d332cbdf.

Revert "Revert "Fix FlexibleSpaceBar Opacity when AppBar.toolbarHeight > ktoolbarHeight (#80453)" (#80545)" (#80589)

Co-authored-by: yusufm <mohsinally@google.com>

This reverts commit f55b2665.