64227 Add null safety migration annotations. (cla: yes, f: material design, framework, waiting for tree to go green)

64235 fix: typo fixed from ENTER to SPACE (cla: yes, d: api docs, f: material design, framework, waiting for tree to go green)

64249 Revert “(insert

move

remove)ChildRenderObject Deprecation: Step 1” (cla: yes, f: cupertino, f: material design, framework)

64254 Re-apply “(insert

move

remove)ChildRenderObject Deprecation: Step 1” (cla: yes, f: cupertino, f: material design, framework)

64268 Fix placement for SnackBarBehavior.floating (a: quality, cla: yes, f: material design, framework, severe: regression, waiting for tree to go green)