Support short-hand object literals {foo, bar} even outside of destructuring.

This is specified in section 12.2.5 of the draft spec (http://people.mozilla.org/~jorendorff/es6-draft.html) by the production |PropertyDefinition : IdentifierReference|, and discussed e.g. in http://ariya.ofilabs.com/2013/02/es6-and-object-literal-property-value-shorthand.html.