introduced by eval.

In the cases where calls to eval have not introduced any variables, we
do not need to perform a runtime call.  Instead, we verify that the
context extension objects have not been created and perform a direct
load.

Not implemented for ARM yet and the scope resolution code could use
some better abstractions.  I'd like to do that in a separate
changelist.
Review URL: http://codereview.chromium.org/20419

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1298 ce2b1a6d-e550-0410-aec6-3dcde31c8c00