After migrating flutter code to null-safety, mock objects not accepting `any`

When assigning the Mock object, it needs to be of the Mock object type, not the BaseClass.

@GenerateMocks(MockSpec<ITransactionRepository>(as: #MockTransactionRepository),
)
void main()
{
    ....
    ITransactionRepository baseObject = MockTransactionRepository();           // wrong
    MockTransactionRepository mockObject = MockTransactionRepository();   // right
    when(baseObject.method(any));     // results in compile error
    when(mockObject.method(any)); // OK
    ...
}

Source: https://github.com/dart-lang/mockito/issues/364