vm: references to context inside objects are not === the original context

Failing test case: ``` js var common = require('../common'); var assert = require('assert'); var vm = require('vm'); var sandbox = {}; sandbox.document = { defaultView: sandbox }; vm.createContext(sandbox); vm.runInContext('var result = document.defaultView === this', sandbox); assert.equal(sandbox.result, true); ``` Investigating this in my local build ... this is the only remaining test suite failure after switching jsdom to io.js vm.