Re: [whatwg/streams] Add helpers to use initial values of Promise methods (#1010)

MattiasBuelens commented on this pull request.



>          function next(done) {
           if (done) {
             resolveLoop();
           } else {
-            pipeStep().then(next, rejectLoop);
+            uponPromise(pipeStep(), next, rejectLoop);

I'm not sure if `uponPromise()` makes this code more readable. I feel like `PerformPromiseThen(pipeStep(), next, rejectLoop)` is more obvious, but then we have to export the low-level `then()` helper again. 😕

This also adds an extra `.catch()` handler on every pipe step, since this is now equivalent to:
```javascript
pipeStep().then(next, rejectLoop).catch(rethrowAssertionErrorRejection);
```
The `.catch()` is not necessary, since we already handle that [here](https://github.com/whatwg/streams/blob/1146228f0bcc1077f6a0c0eff2452b89332ee2ba/reference-implementation/lib/readable-stream.js#L469):
```javascript
setPromiseIsHandledToTrue(pipeLoop());
```
which is equivalent to:
```javascript
pipeLoop().catch(rethrowAssertionErrorRejection);
```
Although you wouldn't want to create this extra promise in a real implementation, I think it's fine for a reference implementation.

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