Replace ButtonBar.bar method with ButtonBarTheme (#37544)

* Added new ButtonBarTheme to replace the deprecated ButtonTheme.bar method.

* Responding to PR feedback.

* [Material] Create material Banner component (#36880)

This PR creates a new material widget for the Banner component. This includes a theme as well. This widget can be dropped into any application, ideally at the top of a listview or scrollview.

(cherry picked from commit 35b6d668e197035ea2b57adb99041542982e8be0)

Removed the use of ButtonTheme.bar in the Banner implementation.

* Updated documentation from PR review comments.

https://dart.googlesource.com/external/github.com/flutter/flutter/+/9dce19e96ff36fc36c50d15f57fa20e096a5b29f
diff --git a/commits.json b/commits.json
index 76370ba..4610626 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"df178fd936c019e0743444f9bb4eece89564c2d4",
-  "external/github.com/flutter/flutter":"ae291745605ce9274a20933e66fffee72cc40a01",
+  "external/github.com/flutter/flutter":"9dce19e96ff36fc36c50d15f57fa20e096a5b29f",
   "sdk":"56b3dede78f91d372a1f573848dd7dcaec84d62f"
 }
\ No newline at end of file