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.

從 rpc-wrapper.js 中第 20 行附近的 ready-event 程式碼開始，追蹤 child worker 如何從 parent worker 初始化。檢查該路徑為何會存取 document，以及 ready event 預期應如何運作。完成標準是 child worker 能夠在不出現 document 錯誤的情況下初始化，同時保留 ready event 及其 IE 相容性。

由索引模型根據 Issue 內容生成。