Remove '!'

Author: fbenevides

test/pusher_channels_flutter_test.dart

@@ -7,12 +7,12 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   setUp(() {
-    TestDefaultBinaryMessengerBinding.instance!.defaultBinaryMessenger.setMockMethodCallHandler(channel, (MethodCall methodCall) async {
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(channel, (MethodCall methodCall) async {
       return '42';
     });
   });
 
   tearDown(() {
-    TestDefaultBinaryMessengerBinding.instance!.defaultBinaryMessenger.setMockMethodCallHandler(channel, null);
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(channel, null);
   });
 }