hangs for me

My code is something like this:

```
function *(next) {
yield next;

this.type = 'text/event-stream; charset=utf-8';
this.set('Cache-Control', 'no-cache');
this.set('Connection', 'keep-alive');

this.body = PassThrough();

let generator = foo();

let result = generator.next();
while (1) {
if (result.done) break;
if (result.value instanceof Promise) {
result = generator.next(yield result.value);
} else {
yield write(this, 'data: '+JSON.stringify(result.value)+'\n\n');
result = generator.next();
}
}

yield write(this, 'event: done\ndata: \n\n');
return this.body.end();
}
```

The generator returns 1 promise, then 10 non-promises, then 1 promise, then 10 non-promises, et cetera. Each promise takes a few hundred milliseconds to resolve.

It works great with 1 or 2 promises, but when I have 3 promises, it hangs on the 28th call to write().