45417 Fix run_release_test flakiness (a: tests, cla: yes, team, team: flakes, waiting for tree to go green)

45431 Support –fast-start for Android applications (as an opt-in) (cla: yes, d: examples, team, tool)

45432 Use RenderSliverPadding to inset SliverFillViewport  (cla: yes, framework)

45438 Handle bare modifier flags on macOS (a: desktop, cla: yes, framework, platform-mac)

45501 implicit-casts:false in flutter/lib/src/animation (cla: yes, framework)

45502 implicit-casts:false in flutter/lib/src/cupertino (cla: yes, f: cupertino, framework, waiting for tree to go green)