I also discovered that our treatment of const declarations is inconsistent
when inside a global eval under 'with' (i.e., when created by
DeclareContextSlots). That is,

  var x;
  eval("const x = 9")

and

  var x;
  eval("with({}) const x = 9")

differ (the former assigns 9, the latter throws). This appears to be an
oversight from earlier changes to our const semantics (the latter shouldn't
throw either). Fixing this is a separate issue, though (and one that doesn't
seem quite worthwhile).

R=mstarzinger@chromium.org
BUG=v8:1991,80591
TEST=

Review URL: https://chromiumcodereview.appspot.com/10067010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11333 ce2b1a6d-e550-0410-aec6-3dcde31c8c00