function BehaviorSubject(value) {
      __super__.call(this);
      this.value = value;
      this.observers = [];
      this.isDisposed = false;
      this.isStopped = false;
      this.hasError = false;
    }
Represents a value that changes over time.
 Observers can subscribe to the subject to receive the last (or initial) value and all subsequent notifications.