66504 Re-land ScaffoldMessenger (a: quality, cla: yes, customer: money (g3), f: material design, framework, severe: new feature, waiting for tree to go green)

66505 Roll Engine from dd35b5b65917 to b0fb2c8a988f (2 revisions) (cla: yes, waiting for tree to go green)

66506 Allow futures that resolve to null (incl. Future) in Future/StreamBuilder (cla: yes, framework)

66507 [flutter_tools] enable single widget reload optimization by default on master (cla: yes, tool, waiting for tree to go green)

66508 [Docs] [Material] Fix Icons api docs (cla: yes, f: material design, framework, team)

66519 Try the Wayland GDK backend, the engine now supports it (cla: yes, tool)