feat(error): check for window and throw error if undefined

Author: eddyerburgh

src/lib/warn-if-no-window.js

@@ -0,0 +1,9 @@
+import { throwError } from './util'
+
+if (typeof window === 'undefined') {
+  throwError(
+    'window is undefined, vue-test-utils needs to be run in a browser environment.\n' +
+    'You can run the tests in node using jsdom + jsdom-global.\n' +
+    'See https://vue-test-utils.vuejs.org/en/guides/common-tips.html for more details.'
+  )
+}

src/mount.js

@@ -1,24 +1,16 @@
 // @flow
 
+import './lib/warn-if-no-window'
 import Vue from 'vue'
 import VueWrapper from './wrappers/vue-wrapper'
 import createInstance from './lib/create-instance'
-import { throwError } from './lib/util'
 import cloneDeep from 'lodash/cloneDeep'
 import createElement from './lib/create-element'
 import './lib/matches-polyfill'
 
 Vue.config.productionTip = false
 
 export default function mount (component: Component, options: Options = {}): VueWrapper {
-  if (!window) {
-    throwError(
-      'window is undefined, vue-test-utils needs to be run in a browser environment.\n' +
-      'You can run the tests in node using jsdom + jsdom-global.\n' +
-      'See https://vue-test-utils.vuejs.org/en/guides/common-tips.html for more details.'
-    )
-  }
-
   const componentToMount = options.clone === false ? component : cloneDeep(component)
   // Remove cached constructor
   delete componentToMount._Ctor

src/shallow.js

@@ -1,5 +1,6 @@
 // @flow
 
+import './lib/warn-if-no-window'
 import Vue from 'vue'
 import cloneDeep from 'lodash/cloneDeep'
 import {