Support defered local check

[MeirionHughes]

Aurelia allows binding to locals that may not exist initially.

example (ref):

<template> 
  <div>
     ${someName.attributes['expanded'].value}
  </div>

   <button ref=someName></button>
<template>

currently this will throw an error that someName is missing from Foo.

requires #106, two-pass support