Re: [whatwg/streams] TransformStream cleanup using "Transformer.cancel" (PR #1283)

> I think calling it before is better.

Ok, I'll change that. This opens up a question on promise handling:

```js
const ts = new TransformStream({
  async cancel(reason) {
    console.log("t.cancel start with", reason);
    await delay(100);
    console.log("t.cancel end");
    throw "t.cancel error";
  },
});
console.log("ts constructed");
ts.readable.cancel("rs.cancel error")
  .then(() => console.log("rs.cancel fulfilled"))
  .catch((err) => console.log("rs.cancel rejected with", err));
ts.writable.abort("ws.abort error")
  .then(() => console.log("ws.abort fulfilled"))
  .catch((err) => console.log("ws.abort rejected with", err));
console.log("started rs.cancel and ws.abort");
```

I think the logical ordering of logs is:

```
# a-1
ts constructed
t.cancel start with rs.cancel error
started rs.cancel and ws.abort
t.cancel end
rs.cancel rejected with t.cancel error
ws.abort rejected with t.cancel error
```

But the following are also possible:

```
# a-2
ts constructed
t.cancel start with rs.cancel error
started rs.cancel and ws.abort
ws.abort fulfilled
t.cancel end
rs.cancel rejected with t.cancel error
```


```
# a-3
ts constructed
t.cancel start with rs.cancel error
started rs.cancel and ws.abort
t.cancel end
rs.cancel fulfilled
ws.abort fulfilled
```

Also consider:

```js
const ts = new TransformStream({
  flush() { console.log("t.flush") },
  async cancel(reason) {
    console.log("t.cancel start with", reason);
    await delay(100);
    console.log("t.cancel end");
    throw "t.cancel error";
  },
});
console.log("ts constructed");
ts.readable.cancel("rs.cancel error")
  .then(() => console.log("rs.cancel fulfilled"))
  .catch((err) => console.log("rs.cancel rejected with", err));
ts.writable.close()
  .then(() => console.log("ws.close fulfilled"))
  .catch((err) => console.log("ws.close rejected with", err));
console.log("started rs.cancel and ws.close");
```

Here similar options are possible:

```
# b-1
ts constructed
t.cancel start with rs.cancel error
started rs.cancel and ws.close
t.cancel end
rs.cancel rejected with t.cancel error
ws.close rejected with t.cancel error
```

```
# b-2
ts constructed
t.cancel start with rs.cancel error
started rs.cancel and ws.close
ws.close fulfilled
t.cancel end
rs.cancel rejected with t.cancel error
```

```
# b-3
ts constructed
t.cancel start with rs.cancel error
started rs.cancel and ws.close
t.cancel end
rs.cancel fulfilled
ws.close fulfilled
```

These each also have effect on whether the error is visible in `writer.closed` or not. In a-1 and b-1 examples `writer.closed` would reject, in a-2, a-3, b-2, and b-3 the it resolves to `undefined`.

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