49511 switching channel prompts to run flutter upgrade (cla: yes, tool, waiting for tree to go green)

49519 [ci] Upgrade to Catalina (cla: yes, waiting for tree to go green)

49520 Roll engine a41ff8d359dc..004cbdf0660a (1 commits) (autoroller: commit, cla: yes)

49523 Roll engine 004cbdf0660a..ac1ba3033c38 (1 commits) (autoroller: commit, cla: yes)

49527 Avoid calling didChangeDependences on a State that has dropped out of the tree (cla: yes, f: focus, framework, waiting for tree to go green)

49554 [flutter_tools] fast start off by default (cla: yes, tool, waiting for tree to go green)