createBus method
Creates a named mix bus routed into parent (the master bus when
null). Names must be unique; "master" is reserved.
Implementation
AudioBus createBus(String name, {AudioBus? parent}) {
if (name == 'master' || _buses.containsKey(name)) {
throw ArgumentError('An audio bus named "$name" already exists.');
}
final bus = onCreateBus(name, parent ?? masterBus);
_buses[name] = bus;
return bus;
}