Standard builtins' prototypes and toString

[C. Scott Ananian]

FWIW, in Chrome 36:

```javascript
> Date.prototype.toString()
"Invalid Date"
> ({ toString: Date.prototype.toString }).toString()
TypeError: this is not a Date object.
> Number.prototype.toString()
"0"
> String.prototype.toString()
""
> Boolean.prototype.toString()
"false"
> RegExp.prototype.toString()
"/(?:)/"
```

That last one is a bit unusual.
 --scott