22942 [web] Introduce flag to enable new rich text implementation (cla: yes, platform-web)

22944 Roll Dart SDK from d75741b19133 to a92baed5f8e2 (1 revision) (cla: yes, waiting for tree to go green)

22945 Fix platform view transforms in CanvasKit (cla: yes, waiting for tree to go green)

22946 [web] Fix drag failure when RMB pointer up event is not received (cla: yes)

22947 Replace g_object_weak_ref with g_object_add_weak_pointer (cla: yes)

22948 [web] Add complex rich text test cases and fix them (cla: yes, platform-web)