30068 Bump buildroot and swiftshader deps (flutter/buildroot#529) (cla: yes)

30069 [Proposal] Improve Canvas Documentation (cla: yes, needs tests)

30073 Roll Skia from c4712cc704e8 to 6dc45289aec0 (2 revisions) (cla: yes, waiting for tree to go green)

30074 Roll Skia from 6dc45289aec0 to 9d74c28e823a (2 revisions) (cla: yes, waiting for tree to go green)

30075 Roll Dart SDK from 9f61c2487bbd to ba5e72e4b2d7 (16 revisions) (cla: yes, waiting for tree to go green)