It would make sense to have these methods call the CheckObjectCoercible() abstract operation on the `this` object, so that they cannot be applied to `null` or `undefined`, just like the other String.prototype methods.
Chrome/V8 and IE/Chakra are the only engines I tested that don’t currently perform the CheckObjectCoercible() step. Here are some relevant bugs:
* Chrome/V8: http://code.google.com/p/v8/issues/detail?id=2218
* IE/Chakra: https://connect.microsoft.com/IE/feedback/details/752807
It would improve interoperability and consistency within the spec if ES6 could define these methods with something like `CheckObjectCoercible(this)` as their first step. Please consider doing so.
That check is already there. It is step 1 of the abstract operation CreateHTML