Support list.reversed in template repeats.

[DanielJoyce]

  <template repeat="{{item in someList.reversed}}">
    ...Stuff...
  </template>

doesn't do anything, whereas this works fine

  <template repeat="{{item in someList}}">
    ...Stuff...
  </template>

So apparently polymer exprssions doesn't support iterable, or reversing a list as far as I can see.