Fallback CL for communication of top-level vars in script blocks

Prototype idea for communication between script blocks - not for review.

Works for toplevel vars and function decls.  Has workarounds for upcaught
exceptions and detecting top-level returns.  Does not work for the following
case:

<script>
  var x = 1;
  function foo() { alert(x); }
</script>
<script>
  var x = 2;
  foo();
</script>