[google_sign_in] Switch unit tests to mock platform implementation (#5703)

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## NEXT
 
-* Fixes tests to recognize new default `forceCodeForRefreshToken` request attribute.
+* Updates tests to use a mock platform instead of relying on default
+  method channel implementation internals.
 
 ## 5.3.1
 

packages/google_sign_in/google_sign_in/pubspec.yaml

@@ -29,13 +29,15 @@ dependencies:
   google_sign_in_web: ^0.10.0
 
 dev_dependencies:
+  build_runner: ^2.1.10
   flutter_driver:
     sdk: flutter
   flutter_test:
     sdk: flutter
   http: ^0.13.0
   integration_test:
     sdk: flutter
+  mockito: ^5.1.0
 
 # The example deliberately includes limited-use secrets.
 false_secrets: