26644 [web] migrate some felt code to null safety (cla: yes, platform-web)

26645 Roll Skia from 917f9193d244 to 26666bda75b9 (1 revision) (cla: yes, waiting for tree to go green)

26646 Roll Skia from 26666bda75b9 to 031d76b67447 (1 revision) (cla: yes, waiting for tree to go green)

26647 [web] Fall-back to a DOM node if Shadow DOM is unavailable. (cla: yes, waiting for tree to go green, platform-web)

26648 Roll Dart SDK from 019918a203bc to 37338b2f4bbb (1 revision) (cla: yes, waiting for tree to go green)