Written file includes webpack-dev-server client modules

Using inline mode, WriteFilePlugin is emitting the webpack-dev-server client entry code into my bundle:

```
// bundle.js

...my bundle...

+ansiHTML.setColors(colors);
+
+function createOverlayIframe(onIframeLoad) {
+ var iframe = document.createElement("iframe");
+ iframe.id = "webpack-dev-server-client-overlay";
+ iframe.src = "about:blank";
+ iframe.style.position
+... about 300k filesize of bundle code
```

I ship this bundle as a standalone artifact so obviously I can't have this emitted. I'd like the convenience of inline mode though; is that possible to support?