Add `start` method to Observers

The TC39 spec has a start method to prevent this scenario from being a problem:

const subscription = range(0, 10000000).subscribe({
  next(value) { 
    if (value > 3) {
      // try to unsubscribe here, I dare you.
      subscription.unsubscribe();
    }
  }
});

The solution was to add an optional start method to Observer:

const subscription = range(0, 10000000).subscribe({
  start(subs) { this.subscription = subs; },
  next(value) { 
    if (value > 3) {
      // this.subscription is literally the same reference you'd get back
      // from the subscribe call itself.
      this.subscription.unsubscribe();
    }
  }
});

[benlesh]

Regardless, I think we’re going to move away from the start method if we use abortSignal: #3122

[lock[bot]]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.