17735 remove allocation from growLTRB; remove dynamism in vector_math (cla: yes)

17736 Remove fuchsia build from cirrus. (cla: yes)

17738 Canvas regression (cla: yes)

17739 Roll src/third_party/dart eb18db2116dc..d5c38cd35486 (18 commits) (cla: yes, waiting for tree to go green)

17740 Update 1.17 engine hash to Dart 2.8.0-20.11.beta (cla: yes)

17741 Roll fuchsia/sdk/core/mac-amd64 from dqerH… to urCsS… (cla: yes, waiting for tree to go green)