17431 Use Metal on iOS by default. (cla: yes)

17433 Started clearing out the semantics information in Dart as well as deleting the AccessibilityBridge. (cla: yes)

17434 Roll src/third_party/dart e8c4aed700f5..021fa0fbd410 (17 commits) (cla: yes, waiting for tree to go green)

17435 Roll src/third_party/skia 65d3ad953957..7a3358c80b6e (20 commits) (cla: yes, waiting for tree to go green)

17436 [fuchsia] Fix bug when applying scale. (cla: yes)

17437 If the application says there is a new texture but does not provide one, reuse the last texture. (cla: yes)