fix(ssr): fix style injection regression

fix vuejs#6603, (regression caused by attempt to fix vuejs#6353)

src/server/bundle-renderer/create-bundle-runner.js

@@ -106,7 +106,7 @@ export function createBundleRunner (entry, files, basedir, runInNewContext) {
     // slightly differently.
     let runner // lazy creation so that errors can be caught by user
     let initialContext
-    return (userContext = {}) => new Promise((resolve, reject) => {
+    return (userContext = {}) => new Promise(resolve => {
       if (!runner) {
         const sandbox = runInNewContext === 'once'
           ? createSandbox()
@@ -126,27 +126,25 @@ export function createBundleRunner (entry, files, basedir, runInNewContext) {
         }
       }
       userContext._registeredComponents = new Set()
+
       // vue-style-loader styles imported outside of component lifecycle hooks
       if (initialContext._styles) {
         userContext._styles = deepClone(initialContext._styles)
-      }
-      // #6353 after the app is resolved, if the userContext doesn't have a
-      // styles property, it means the app doesn't have any lifecycle-injected
-      // styles, so vue-style-loader never defined the styles getter.
-      // just expose the same styles from the initialContext.
-      const exposeStylesAndResolve = app => {
-        if (!userContext.hasOwnProperty('styles')) {
-          userContext.styles = initialContext.styles
+        // #6353 ensure "styles" is exposed even if no styles are injected
+        // in component lifecycles.
+        // the renderStyles fn is exposed by vue-style-loader >= 3.0.3
+        const renderStyles = initialContext._renderStyles
+        if (renderStyles) {
+          Object.defineProperty(userContext, 'styles', {
+            enumerable: true,
+            get () {
+              return renderStyles(userContext._styles)
+            }
+          })
         }
-        resolve(app)
       }
 
-      const res = runner(userContext)
-      if (typeof res.then === 'function') {
-        res.then(exposeStylesAndResolve).catch(reject)
-      } else {
-        exposeStylesAndResolve(res)
-      }
+      resolve(runner(userContext))
     })
   }
 }