18877 Roll Fuchsia Linux SDK from j9T0m… to U7i_W… (cla: yes, waiting for tree to go green)

18878 Refactor Win32FlutterWindow in preparation for UWP windowing implementation (cla: yes, platform-windows)

18879 Roll Dart SDK from 48b4528a44ac to 87c5e3612b70 (1 revision) (cla: yes, waiting for tree to go green)

18880 nullability clean-ups (cla: yes)

18881 Roll Fuchsia Mac SDK from Xqev9… to uCq3V… (cla: yes, waiting for tree to go green)

18885 Add scroll event support (cla: yes)

18886 Update FlTextInputPlugin method handler style. (cla: yes)