Add tests for FlBinaryMessenger, FlBasicMessageChannel, FlMethodChannel (#18597)

* Add mock implementations of the Flutter embedding API and EGL
* Add tests for FlBinaryMessenger, FlBasicMessageChannel, FlMethodChannel
https://dart.googlesource.com/external/github.com/flutter/engine/+/a095cc23d8d5a539fd6179846875d7218c60bdff
diff --git a/commits.json b/commits.json
index a530fe2..e110c23 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"2bd95d9a4c85e8aef670b849bdd2c8145666b216",
+  "external/github.com/flutter/engine":"a095cc23d8d5a539fd6179846875d7218c60bdff",
   "external/github.com/flutter/flutter":"5663e02645a6e69fedfe8553cbe7940cbb592be9",
   "sdk":"521acb17e477ca02d61fa89800cfdda949e362eb"
 }
\ No newline at end of file