68150 Migrate some widget tests to NNBD (a: accessibility, cla: yes, framework, waiting for tree to go green)

68151 Updated Builder API doc (cla: yes, framework, waiting for tree to go green)

68156 [devicelab] allow the devicelab to take a screenshot if the iOS connection fails with FLUTTER_IOS_SCREENSHOT_ON_CONNECTION_FAILURE (cla: yes, team, tool)

68157 [NNBD] Migrate some Widgets tests (a: null-safety, a: tests, cla: yes, framework, team, waiting for tree to go green)

68158 revert engine to 11d756a62ed0ddf87a9ce20b219b55300ec6b67d (cla: yes, engine)