27838 Migrate all Python hashbangs to Python 3 (cla: yes, platform-fuchsia, needs tests)

27840 Roll Dart SDK from 2e22cdf76836 to 2cddb14dc9a9 (2 revisions) (cla: yes, waiting for tree to go green)

27841 Roll Skia from 62d42db2829d to 028e45b2f013 (1 revision) (cla: yes, waiting for tree to go green)

27842 Roll Skia from 028e45b2f013 to 464703f5148a (1 revision) (cla: yes, waiting for tree to go green)