Macos systemnavigator pop (#9019)

Adds the flutter/platform channel to the macOS shell, and implements
SystemNavigator.pop.

Other methods from this channel will be implement in the future.

macOS part of flutter/flutter#30713
https://dart.googlesource.com/external/github.com/flutter/engine/+/dbf083bda15740c6c10891a402c81c4ebf67e0fd
diff --git a/commits.json b/commits.json
index f90fe6f..8c4a66d 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"760fea25cf41aa2541f29a3123eabacd92a5a204",
+  "external/github.com/flutter/engine":"dbf083bda15740c6c10891a402c81c4ebf67e0fd",
   "external/github.com/flutter/flutter":"90f38907d70481970046652c61565f896f40595b",
   "sdk":"2fd1021326d7f2c6da68017f90e6e6523c615e05"
 }
\ No newline at end of file