vm: strict mode ReferenceError with assignment to context property

Split off from https://github.com/nodejs/node/issues/5344#issuecomment-281392069.

```js
'use strict';
const vm = require('vm');
const ctx = vm.createContext({ x: 42 });

try {
  const result = vm.runInContext('"use strict"; x = 1', ctx);
} catch(e) {
  console.log(e.stack);
  console.log('x is', ctx.x)
}
```