9789 fix ColorFilter.matrix constness (cla: yes)

9791 Roll Wuffs and buildroot (cla: yes)

9792 Update flutter_web to latest (cla: yes)

9793 Fix typo in PlaceholderAlignment docs (cla: yes)

9797 Remove breaking asserts (cla: yes)

9799 Update buildroot to c4df4a7b to pull in MSVC 2017 Update 9 on Windows. (cla: yes)

9808 Document FontFeature class (cla: yes)

9809 Document //flutter/shell/common/rasterizer (cla: yes)

9812 Roll src/third_party/dart 24725a8559..28f95fcd24 (32 commits) (cla: yes)