Support Object Short Notation

This adds support for object short notation:

```javascript
function f(x, y) { return {x: x, y: y}; }
// Object Short Notation
function f(x, y) { return {x, y}; }
```

Support for this feature is limited to `esnext` mode.
Merge pull request #1731 from yungsters/master

Support Object Short Notation