fix: compilation errors from invalid/unknown identifiers (#579)

Author: mrzwk-b

packages/firebase_ui_oauth/lib/src/oauth_provider_button_style.dart

@@ -2,8 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:flutter/widgets.dart';
-
+import 'package:flutter/material.dart';
 import 'package:firebase_ui_shared/firebase_ui_shared.dart';
 import 'package:flutter_svg/svg.dart';
 

packages/firebase_ui_shared/lib/src/themed.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:flutter/widgets.dart';
+import 'package:flutter/material.dart';
 
 /// {@template ui.shared.themed_value}
 /// Helper class that helps to resolve a value based on the current app