* Let webpack2 to handle ES2015 module system
Since Node.js can't do that, we need to transpile
ES2015 module system in the emit-file-loader.
* Use sourceMaps only in dev.
* Introduce a transform option to emit-file-loader
So, we can move our ES2015 transpile code with that option.
* Remove unwanted argument options.
* Update comments.
* Use dev flag instead of NODE_ENV
* add detach-plugin
* detach-plugin: remove unused property
* watch-pages-plugin: replace _error.js when user defined one was added/removed
* dynamic-entry-plugin: delete cache
* fix HMR settings for _error.js
* render: pass error only on dev
* hot-reload: enable to hot-reload error page
* server: check if /_error has compilation errors
* webapck-dev-client: fix reloading /_error