28222 Manual engine roll to 6d7eb52185b117a3972cac4e23625f97198114d9 (cla: yes)

28224 Adds a template for Pull Requests (cla: yes, team, waiting for tree to go green)

28235 Remove line breaks from PR template (cla: yes, team)

28238 Remove set literal syntax (cla: yes, team)

28265 Revert “Remove unused –packages argument to gen_snapshot.” (cla: yes, tool)

28272 Make logcat less chatty on perf tests (cla: yes, team)

PRs closed in this release of flutter/engine