This will allow us to move expressions from one function to another, for
example when the parser determines that a given cover grammar instance
is actually the default value initializer for an arrow function.

R=svenpanne@chromium.org, marja@chromium.org
BUG=

Review URL: https://codereview.chromium.org/636403003

Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24757 ce2b1a6d-e550-0410-aec6-3dcde31c8c00