Unlink only pages bundle in dev env (#4015)

* Unlink only pages bundle in dev env

* Use IS_BUNDLED_PAGE regexp for unlink file plugin

server/build/plugins/unlink-file-plugin.js

 import { join } from 'path'
 import { unlink } from 'mz/fs'
+import { IS_BUNDLED_PAGE } from '../../utils'
 
 export default class UnlinkFilePlugin {
   constructor () {
@@ -9,7 +10,7 @@ export default class UnlinkFilePlugin {
   apply (compiler) {
     compiler.plugin('after-emit', (compilation, callback) => {
       const removed = Object.keys(this.prevAssets)
-      .filter((a) => !compilation.assets[a])
+      .filter((a) => IS_BUNDLED_PAGE.test(a) && !compilation.assets[a])
 
       this.prevAssets = compilation.assets