Subject<T>  constructor 
      
      Subject<T> (
    
- StreamController<T> controller,
- Stream<T> stream
Constructs a Subject which wraps the provided controller.
This constructor is applicable only for classes that extend Subject.
To guarantee the contract of a Subject, the controller must be
a broadcast StreamController and the stream must also be a broadcast Stream.
Implementation
Subject(StreamController<T> controller, Stream<T> stream)
    : _controller = controller,
      assert(stream.isBroadcast, 'Subject requires a broadcast stream'),
      super(stream);