WIP - Make named yield check async.
WIP - Make named yield check async.

- Per Tom's comment, we don't need to make sure that the named yield exists at
  the time of calling setTemplate. But if it doesn't exist after the flush, it
  probably means the user forgot to declare the yield so we'll give them a
  helpful warning message. It's just a warning message and not an error. We
  could set varying degrees of warning messages at some point.

TechplexEngineer/iron-router,sean-stanley/iron-router,assets1975/iron-router,iron-meteor/iron-router,Hyparman/iron-router,TechplexEngineer/iron-router,snamoah/iron-router,Aaron1992/iron-router,Hyparman/iron-router,DanielDornhardt/iron-router-2,appoets/iron-router,jg3526/iron-router,snamoah/iron-router,Sombressoul/iron-router,DanielDornhardt/iron-router-2,leoetlino/iron-router,abhiaiyer91/iron-router,firdausramlan/iron-router,ecuanaso/iron-router,Sombressoul/iron-router,iron-meteor/iron-router,tianzhihen/iron-router,tianzhihen/iron-router,Aaron1992/iron-router,abhiaiyer91/iron-router,firdausramlan/iron-router,jg3526/iron-router,appoets/iron-router,sean-stanley/iron-router,ecuanaso/iron-router,assets1975/iron-router