72829 Migrate some sample code to NNBD [3] (a: null-safety, cla: yes, d: api docs, d: examples, documentation, f: cupertino, framework, team, waiting for tree to go green)

72837 [NNBD] Migrate sample code pt 3 (a: null-safety, cla: yes, f: material design, framework, team, waiting for tree to go green)

72939 Complete migration to null safety in api documentation (a: null-safety, cla: yes, f: cupertino, f: material design, framework, team)

73047 [web] make sure tests and benchmarks use correct backend - add autodetect false (cla: yes, team)

73052 Avoid broken symlinks in embedded Flutter frameworks (cla: yes, platform-mac, team, tool, waiting for tree to go green)