Make onSnapshot work with rxjs observers

[Feiyang1]

It fixes an issue where passing rxjs Observer to onSnapshot causes error Error: Uncaught TypeError: this._next is not a function.
For example,

const data$ = rxjs.Observable.create(function subscribe(observer) {
  firebase.firestore().collection('test').onSnapshot(observer);
});

data$.subscribe({
  next: data => handleData(data)
});

A similar issue is being fixed for FCM - #3221

[changeset-bot]

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Latest commit: 1a7a3b5

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Binary Size Report

Affected SDKs

• @firebase/firestore

  Type	Base (4d1712d)	Head (d6d98d9)	Diff
  browser	248 kB	248 kB	+252 B (+0.1%)
  esm2017	193 kB	194 kB	+260 B (+0.1%)
  main	493 kB	493 kB	+350 B (+0.1%)
  module	246 kB	246 kB	+252 B (+0.1%)

• @firebase/firestore/memory

  Type	Base (4d1712d)	Head (d6d98d9)	Diff
  browser	186 kB	186 kB	+252 B (+0.1%)
  esm2017	145 kB	145 kB	+260 B (+0.2%)
  main	362 kB	363 kB	+350 B (+0.1%)
  module	184 kB	184 kB	+252 B (+0.1%)

• firebase

  Type	Base (4d1712d)	Head (d6d98d9)	Diff
  firebase-firestore.js	286 kB	286 kB	+252 B (+0.1%)
  firebase-firestore.memory.js	225 kB	226 kB	+252 B (+0.1%)
  firebase.js	819 kB	820 kB	+252 B (+0.0%)

Test Logs

• Base (4d1712db): https://github.com/firebase/firebase-js-sdk/actions/runs/151868401
• Head (d6d98d97): https://github.com/firebase/firebase-js-sdk/actions/runs/152063703

[schmidt-sebastian]

FWIW, the change that broke this was merged on Friday and never released.

[Feiyang1]

which one is it?

[schmidt-sebastian]

https://github.com/firebase/firebase-js-sdk/pull/3294/files#diff-9cfbf72c75b0e1f9df5069543a6bced1R1172

[Feiyang1]

Cool. Changed to an empty changeset.