ES6: Implement object literal property shorthand

This allows the following:

var x = 1;
var o = {x};

This is under the --harmony-object-literals flag.

BUG=v8:3584
LOG=y
R=marja@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/584993002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24291 ce2b1a6d-e550-0410-aec6-3dcde31c8c00