Only dismiss dropdowns if the orientation changes, not the size. (#42482)

This changes the DropDownButton so that instead of dismissing itself when any metrics change occurs, it only dismisses itself when the orientation changes.

This gets around the fact that we can't currently have a dropdown and a text field on the same page because the keyboard disappearing when the dropdown gets focus causes a metrics change, and the dropdown immediately disappears when activated.

It still will cause the keyboard to jump up and down between controls, but that's a larger issue. At least now we can use the two together again.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/dee75839087fb9cdfab342dd26587caa51b31eaf
diff --git a/commits.json b/commits.json
index 62aa5f3..d56f919 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"e3b5d8ef32266326d5de3f43f8ceab46095ee72e",
-  "external/github.com/flutter/flutter":"9423a01204ba9abf2e4b26fda3d5ef06d6897ef9",
+  "external/github.com/flutter/flutter":"dee75839087fb9cdfab342dd26587caa51b31eaf",
   "sdk":"892fcf2c4586bc00a23c19ddfd8be0064681ad05"
 }
\ No newline at end of file