29619 make literals const for @immutable constructors (cla: yes, team)

29621 Update PULL_REQUEST_TEMPLATE.md (cla: yes, team)

29623 Revert “Reland composite physical layers on all platforms” (cla: yes, framework)

29625 Fix typo in flutter_tools (cla: yes, tool)

29627 Manual engine roll for 2019-03-19 (cla: yes)

29630 Add heart shapes to CupertinoIcons (cla: yes, f: cupertino, framework)

29632 Enable platform views for Flutter Gallery on iOS. (cla: yes, team, team: gallery)

29633 Download secondary SDK (cla: yes, tool)