Make the binding map be an object and not an array

This is a work around for a JSC bug when using arrays with hole in them
it sometimes "removes" the hole, changing [/* hole */, value] to
[value]. Thisonly happens in optimized mode. If the web inspector is
open we do not hit this bug.

BUG=https://github.com/Polymer/ShadowDOM/issues/203
TBR=rafaelw

Review URL: https://codereview.appspot.com/12580046