make router loadable in node

Author: yyx990803

src/index.js

@@ -23,7 +23,7 @@ Router.install = function (Vue) {
 
 // auto install
 /* istanbul ignore if */
-if (window.Vue) {
+if (typeof window !== 'undefined' && window.Vue) {
   Router.install(window.Vue)
 }
 

src/router/index.js

@@ -1,3 +1,4 @@
+var util = require('../util')
 var Recognizer = require('route-recognizer')
 var historyBackends = {
   abstract: require('../history/abstract'),
@@ -64,7 +65,8 @@ function Router (options) {
   this._suppress = !!options.suppressTransitionError
 
   // create history object
-  this.mode = this._abstract
+  var inBrowser = util.Vue.util.inBrowser
+  this.mode = (!inBrowser || this._abstract)
     ? 'abstract'
     : this._history
       ? 'html5'