26247 Refactor: Group together externally-managed UIDartState. (cla: yes)

26248 Roll Skia from d7d7a8215e6c to a65c295c1c9b (1 revision) (cla: yes, waiting for tree to go green)

26252 Roll Skia from a65c295c1c9b to 814c6db4c04d (1 revision) (cla: yes, waiting for tree to go green)

26253 suppress output from intentionally failing tests (cla: yes, platform-web)

26254 Use ‘pub get –offline’ for scenario_app deps (cla: yes)

26257 updated the documentation for message loops a bit (cla: yes, waiting for tree to go green)