MenuAnchor hover traversal fixes (#150914)

Fixes https://github.com/flutter/flutter/issues/150910 and https://github.com/flutter/flutter/issues/150911.

https://github.com/flutter/flutter/issues/150910 is fixed by invalidating the focus scope whenever a hover occurs. I'm interested to hear of better fixes -- it feels a bit extreme to invalidate the focus scope so often.

https://github.com/flutter/flutter/issues/150911 is fixed by replacing TextButton.onHover with MouseRegion.onHover and MouseRegion.onExit. The issue appears to be that MouseRegion.onEnter is called on scroll, whereas MouseRegion.onHover is not. I'm not confident this is a great solution, so please let me know if you all have any suggestions!

@Piinks @dkwingsmt
https://dart.googlesource.com/external/github.com/flutter/flutter/+/1f4c6ebc972a73a60b4bdeb331c8b5137bd95a3d

DEPS

diff --git a/DEPS b/DEPS
index adb618b..cc2902f 100644
--- a/DEPS
+++ b/DEPS
@@ -284,7 +284,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'b62e1e6902d27e260bce45b154bb42237f26fb60',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '0f7bceb9c4d49d95f73effd9b0b593a95cc68f9c',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '1f4c6ebc972a73a60b4bdeb331c8b5137bd95a3d',
 
   'engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index 86f6709..8d7a409 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"317eb61076460bc9f8c4cbfccb714ecfe0072b70",
   "engine/src/flutter/third_party/dart":"b62e1e6902d27e260bce45b154bb42237f26fb60",
-  "flutter":"0f7bceb9c4d49d95f73effd9b0b593a95cc68f9c"
+  "flutter":"1f4c6ebc972a73a60b4bdeb331c8b5137bd95a3d"
 }
\ No newline at end of file