streamsapi.js externs: Sink / Source types too strict?

All the Promise-returning methods for ReadableStreamSource and ReadableStreamSink can actually return thenables.

This means that, for example, the return type specified here is too strict:

``` javascript
/** @type {(undefined|function(*):(!Promise<*>|undefined))} */
ReadableStreamSource.prototype.cancel;
```

I haven't actually had a real problem with this, I just work on the Streams standard. It may well be that there's no need to make this requirement more relaxed.