Generator objects have [[Class]] === "Generator"

Generator object maps now link to their constructors, which are created
with a "Generator" class name.  This does not cause a per-generator
constructor property to be set.

BUG=v8:2355
TEST=mjsunit/harmony/generators-objects

Review URL: https://codereview.chromium.org/14262004

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14309 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Add d8 functionality for switching between realms (a.k.a. contexts)

R=mstarzinger@chromium.org,yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/14295011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14310 ce2b1a6d-e550-0410-aec6-3dcde31c8c00