35499 Added MaterialApp.themeMode to control which theme is used. (cla: yes, f: material design, framework)

35548 Various doc fixes (cla: yes, framework)

35556 ios (iPhone6) and iPhone XS tiles_scroll_perf tests (cla: yes, severe: performance, team, ⌺‬ platform-ios)

35560 Support for elevation based dark theme overlay color in the Material widget (cla: yes, f: material design, framework)

35573 update packages (cla: yes, team)

35574 Fix semantics for floating pinned sliver app bar (a: accessibility, cla: yes, f: scrolling, framework, waiting for tree to go green)

35646 Prepare for Socket implements Stream (cla: yes)