feat(server, webpack-plugin): webpack 4 support (vuejs#7839)

SSR webpack 4 compat

Author: pi0

Diff for: src/server/webpack-plugin/client.js

@@ -1,6 +1,6 @@
 const hash = require('hash-sum')
 const uniq = require('lodash.uniq')
-import { isJS, isCSS } from './util'
+import { isJS, isCSS, onEmit } from './util'
 
 export default class VueSSRClientPlugin {
   constructor (options = {}) {
@@ -10,7 +10,7 @@ export default class VueSSRClientPlugin {
   }
 
   apply (compiler) {
-    compiler.plugin('emit', (compilation, cb) => {
+    onEmit(compiler, 'vue-client-plugin', (compilation, cb) => {
       const stats = compilation.getStats().toJson()
 
       const allFiles = uniq(stats.assets

Diff for: src/server/webpack-plugin/server.js

@@ -1,4 +1,4 @@
-import { validate, isJS } from './util'
+import { validate, isJS, onEmit } from './util'
 
 export default class VueSSRServerPlugin {
   constructor (options = {}) {
@@ -10,7 +10,7 @@ export default class VueSSRServerPlugin {
   apply (compiler) {
     validate(compiler)
 
-    compiler.plugin('emit', (compilation, cb) => {
+    onEmit(compiler, 'vue-server-plugin', (compilation, cb) => {
       const stats = compilation.getStats().toJson()
       const entryName = Object.keys(stats.entrypoints)[0]
       const entryInfo = stats.entrypoints[entryName]

Diff for: src/server/webpack-plugin/util.js

@@ -21,4 +21,14 @@ export const validate = compiler => {
   }
 }
 
+export const onEmit = (compiler, name, hook) => {
+  if (compiler.hooks) {
+    // Webpack >= 4.0.0
+    compiler.hooks.emit.tapAsync(name, hook)
+  } else {
+    // Webpack < 4.0.0
+    compiler.plugin('emit', hook)
+  }
+}
+
 export { isJS, isCSS } from '../util'