generator.

Contexts are no longer stored in the AST but in the code generator's
state.  This means that the running the code generator selector is not
required to use the toplevel code generator (for instance, if we
already know that we can and should use it).

Review URL: http://codereview.chromium.org/546075

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3645 ce2b1a6d-e550-0410-aec6-3dcde31c8c00