Revamp Async Generator Error handling

[brainkim]

Description:
In 0.7, async generator components which throw errors can cause unhandled promise rejections, even if the errors are caught by parent executions. For instance:

async function *Child() {
  for await ({children} of this) {
    throw new Error("this is handled");
  }
}

try {
  await renderer.render(<Child />, document.body);
} catch (err) {
  // renderer.render throws the error but an unhandled promise rejection still happens internally.
}

This issue has affected Crank in various forms for a while now.

The tricky thing is:

• errors which happen in the course of rendering should cause parent render() and refresh() calls to throw.

• because these calls throw, there should be no unhandled promise rejections.

• errors which happen because an async generator component continues after a yield should cause an unhandled promise rejection, because there is no call that can throw for this execution.

• these cases should be handled by parent sync/async generator components.

• errors which happen during unmounting should not be catchable by parent components. They should cause unhandled promise rejections instead.

[brainkim]

I’ve almost got this figured out! In the latest commits, the only spurious unhandled rejections are those thrown by an async generator component rethrowing an error. Not a big deal, but I’m curious why this is the case.

Edit:

it seems to be because the component which is rethrowing an error is diffing its children an additional time, but before the rethrown error is handled. I’m too lazy to figure this out right now.