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.

Fixes gh-686

Fixes gh-694

* Improve tests for GeneratorPrototype methods

- Assert return values more consistently
- Expand tests for constraints on `this` value

* Add more tests for dynamic GeneratorFunctions

* Add more tests for GenerationFunction.prototype

* Add more tests for the GeneratorFunction object

* Extend test: GeneratorFunction.prototype.prototype

* Improve precision of tests for generator methods

Extend existing assertions to explicitly verify that execution halts at
the intended location. Correct tests which were previously asserting
this behavior in contexts that did not match their name/description.

* Remove unused variables

* fixup! Improve tests for GeneratorPrototype methods

* fixup! Improve tests for GeneratorPrototype methods

* Add missing test for early error

* Add missing test for WithBaseObject

* Improve coverage for `new.target`

* Add test for deletion of SuperReference

* Add tests for `in` keyword restrictions

* fixup! Improve coverage for `new.target`

The global isNaN is not precise at all, and Number.isNaN is an ES6 feature that makes it preferrable to use assert's sameValue for NaN values, as it handles it internally using the comparison.

* Improve test coverage for `super` keyword

Add tests for SuperCall and SuperProperty, organized together in the
`test/language/expressions/super/` directory. For SuperProperty, include
tests for usage from within Object initializers and class bodies because
a different set of semantics are observable from each context.

Extend test for HTML comments

The V8 engine incorrectly requires a leading newline character for
MultiLineComments which contain the optional trailing HTMLCloseComment
[1]. Extend the current tests to fail when such an invalid restriction
is in place.
[1] https://bugs.chromium.org/p/v8/issues/detail?id=5142

Switch to non-positional argument.

A subtle aspect of the for-of iteration protocol concerns abrupt
completions that do *not* trigger iterator closing. Although this detail
is implicit in the current structure of the specification text, some
hosts may violate the protocol by closing the iterator because later
steps *do* specify that behavior.

The V8 engine is one such host--as of this writing, it incorrectly
closes the iterator when accessing the `value` property of the iterator
result produces an abrupt completion.

Add tests verifying that the iterator protocol is not violated in this
way for abrupt completions during the semantics of for-of evaluation.

ConversionError and RegExpError tests are methods that existed on
something that preceeded a IE9 Standard. They do not exist in the
specs, at least since ES5.

It's not test262 responsibility to maintain these tests as these
features can be freely implemented by any runtime.

and Number.prototype

Relax test for forbidden extension

Remove unused variables