72803 [text_input] prepare for custom text input sources (cla: yes, framework, waiting for tree to go green)

72817 Cherrypick engine to 63a92ca0db5f5db462714f60f5b8e88bcef8f57b (cla: yes, engine)

72827 Roll Engine from af6889a600e7 to b7cd3f8e706d (22 revisions) (cla: yes, waiting for tree to go green)

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)