22443 [web] Refactor _measureSubstring to better suit rich text measurement (cla: yes, platform-web)

22444 [web] Reuse the existing font string builer in TextStyle (cla: yes, platform-web)

22445 fuchsia: Update buildroot (cla: yes, platform-fuchsia)

22446 Make CkPath resurrectable (cla: yes, waiting for tree to go green)

22447 Roll Fuchsia Mac SDK from e-4Jm-yWa… to 9t3yDRxI8… (cla: yes, waiting for tree to go green)

22449 [flutter_releases] Flutter 1.22.4 engine cherrypicks (cla: yes, platform-android, platform-ios)