64050 1.21.0-9.1.pre cherrypicks (cla: yes, engine, f: material design, framework, team, tool)

64065 [flutter_tools] add more info to coverage script (cla: yes, tool, waiting for tree to go green)

64160 [null-safety] roll to null-safe platform (cla: yes, team, tool)

64166 Add WindowProc delegation to Windows template (cla: yes, tool)

64173 Update Windows template for wrapper file change (cla: yes, tool)

64181 Move v1 embedding migration warning from plugin consumers to all apps (cla: yes, tool)

64186 [flutter_tools] null assertions off by default for web (cla: yes, team, tool)