Re: [whatwg/streams] What would happen if the data stream "pauses" then continues (see mode detail below) (#1008)

The important bit is that `append()` resolves the promise returned by `waitForData()`. You can't have `waitForData()` loop until `this.source.length` changes, since *nothing else can run* while that loop is going.

When you use `new Promise()`, the return value of the callback is ignored. Therefore, `return this.waitForData()` will not "chain" the original promise with the returned promise. (I know, it can be a bit confusing since `.then()` does allow you to return a promise.)

One possible solution could be:
```javascript
{
    waitForData() {
        this.nextDataPromise = new Promise((resolve) => {
            this.nextDataResolve = resolve;
        });
        if (this.position < this.source.length) {
            this.nextDataResolve();
        }
        return this.nextDataPromise;
    }

    append(newChunks) {
        this.source.push(...newChunks);
        if (this.nextDataPromise && this.position < this.source.length) {
            this.nextDataResolve();
        }
    }
}
```

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