Fixes gh-696
As pointed in gh-696, the ArrayBuffer ctor won't throw a RangeError when
invoked without arguments.
Instead of fixing the invalid assertion, this commit removes it as the
coverage for subclassing is already satisfied by the use of slice.
The latest revision of ECMA262 makes special provisions for classes
which extend the `null` value [1]. Update the relevant tests
accordingly.
[1] https://github.com/tc39/ecma262/issues/543
