Error when initializing Child Workers

I tried initializing child web workers from a parent worker, and it seems to work well with the exception of the following error being thrown:

```
rpc-wrapper.js:20 Uncaught ReferenceError: document is not defined
at Worker. (rpc-wrapper.js:20)
```

Digging in, it seems like its part of building up the `ready` event, although document is not in scope in the context of a WebWorker.

```
let evt = document.createEvent('Event');
evt.initEvent(d.method, false, false);
evt.data = d.params;
worker.dispatchEvent(evt);
```

What is the purpose of the `ready` event? Is there a way to build the event in a WebWorker-safe manner? I've looked into `new Event`, but that's not IE compatible.

Start in rpc-wrapper.js at the ready-event code around line 20 and trace how child workers are initialized from a parent worker. Check why document is accessed in that path and how the ready event is expected to behave. Done means child workers initialize without the document error while preserving the ready event and its IE compatibility.

Written by the indexing model from the issue text.