All module expressions, and all variables that might refer to modules,
are assigned interfaces (module types) that are resolved using
unification. This is necessary to deal with the highly recursive
nature of ES6 modules, which does not allow any kind of bottom-up
strategy for resolving module names and paths.

Error messages are rudimental right now. Probably need to track
more information to make them nicer.

R=svenpanne@chromium.org
BUG=v8:1569
TEST=

Review URL: https://chromiumcodereview.appspot.com/9615009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10966 ce2b1a6d-e550-0410-aec6-3dcde31c8c00