We need our own anonymous ID for Flutter App

We are using this for Flutter App. 

I am looking for like `setAnonymousId()` method. But, I notice there is no support for this function in the current version of `analytics_flutter`.
In our app, we are generating the anonymousId, so we want to reuse this for the identify() call. 

It would be good to extend the identify() or another setAnonymousId() method for the analytics. 
