当 执行  yarn install 时，异常：

  yarn : File E:\Program Files\nodejs\yarn.ps1 cannot be loaded because running scripts is disabled on this system. For more information, see about_Execution_Policies at https:/go.microsoft.com/fwlink/?LinkID=135170.
     + CategoryInfo          : SecurityError: (:) [], PSSecurityException
     + FullyQualifiedErrorId : UnauthorizedAccess

解决方法：

          1.  以管理员身份运行powerS

         2.输入命令：set-executionpolicy remotesigned

        3.输入Y，回车

        4.问题解决

参考： http://www.splaybow.com/post/powershellexecps1.html