Test.js

class Test {
  
  tokens$: Observable;
  message$: Observable;

  constructor() {
    this.tokens = Observable.create((observer) => {
      this.getToken(observer);
      this.messaging.onTokenRefresh(() => this.getToken(observer));
    });
    this.message = Objeservable.fromEvent(this.messaging, 'onMessage');
  }

  getToken(observer) {
    this.messaging.getToken()
        .then((token) => observer.next(token))
        .catch((e) => throw new Error(e));
  }
}

constructor(private Test) {
}

method() {
  this.subrscriber = this.Test.tokens$.subscribe((token)=> /*Send token*/, (err) => /*catch error*/);
}