only inject when __inject__ is exposed (e.g. when used with extract-text-webpack-plugin)

Author: yyx990803

lib/loader.js

@@ -295,6 +295,9 @@ module.exports = function (content) {
     if (needsHotReload) {
       styleInjectionCode += `  if (disposed) return\n`
     }
+    if (isServer) {
+      styleInjectionCode += `var i\n`
+    }
     parts.styles.forEach(function (style, i) {
       // require style
       var requireString = style.src
@@ -306,7 +309,7 @@ module.exports = function (content) {
       // vue-style-loader exposes inject functions during SSR so they are
       // always called
       var invokeStyle = isServer && hasVueStyleLoader
-        ? code => `  ;(${code}).__inject__(ssrContext)\n`
+        ? code => `;(i=${code},i.__inject__&&i.__inject__(ssrContext),i)\n`
         : code => `  ${code}\n`
 
       var moduleName = (style.module === true) ? '$style' : style.module