Fixes issue 4512.  In JsStackEmulation, avoid rewriting references to
non-references.  For example, don't rewrite bar['foo']() to
(line="123",bar['foo'])(), because that will result in
"this" being set incorrectly in the invoked function.


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7516 8db76d5a-ed1c-0410-87a9-c151d255dfc7