Add unregisterMethod for Server

[jeiea]

Motivation

While writing my flutter app's initialization / cleanup state, I found this.

Currently registerMethod throws if same method name is registered already.

  void registerMethod(String name, Function callback) {
    if (_methods.containsKey(name)) {
      throw ArgumentError('There\'s already a method named "$name".');
    }

    _methods[name] = callback;
  }

I thought about writing code to unregister and re-register methods based on the state of the app. But it wasn't easy due to this.

Can this be considered?

Alternatives

I think registerMethod may return disposer callback.

[natebosch]

I don't know all the potential impacts of a unregisterMethod. I think a safer change could be to add an overrideMethod which skips the contains(key), though I haven't thought through all the implications, or whether adding method is breaking change.