For asynchronous tests, the contract between test file and test runner
is implicit: runners are expected to inspect the source code for
references to a global `$DONE` identifier.

Promote a more explicit contract between test file and test runner by
introducing a new frontmatter "tag", `async`. This brings asynchronous
test configuration in-line with other configuration mechanisms and also
provides a more natural means of test filtering.

The modifications to test files was made programatically using the
`grep` and `sed` utilities:

    $ grep "\$DONE" test/ -r --files-with-match --null | \
        xargs -0 sed -i 's/^\(flags:\s*\)\[/\1[async, /g'
    $ grep "\$DONE" test/ -rl --null | \
        xargs -0 grep -E '^flags:' --files-without-match --null | \
        xargs -0 sed -i 's/^---\*\//flags: [async]\n---*\//'

Remove files that tested both PerformPromiseThen and
PromiseResolveFunction in favor of new tests that test
PromiseResolveFunction more directly and completely.

This allows these tests to be more easily reused to test Promise
implementations in isolation.

- RegExp.prototype[Symbol.split] calls ToUint32 (https://github.com/tc39/ecma262/issues/92)
- Species lookup removed from Promise.all and Promise.race (https://github.com/tc39/ecma262/issues/151)
- Generator functions are no longer constructors (https://github.com/tc39/ecma262/pull/171)

Fixes #444

The `negative` frontmatter tag expresses an expectation for the behavior
of the test file as a whole. The `assert.throws` helper function offers
more fine-grained control over expectations because it may be applied to
specific statements and expressions. This makes it preferable in cases
where it may be used (i.e. when the test body does not describe a syntax
error or early error).

Re-implement assertions for errors to use the `assert.throws` helper
function wherever possible.

Uniformly and exhaustively test the generic aspects of the static- and
instance Promise methods, including:

- function `length` properties
- function `name` properties
- method property descriptors

This test's description concerns the behavior of `Promise.all` when the
IteratorStep abstract operation fails due to an abrupt completion
returned by the iterator's `next` method. The test body did not actually
assert that functionality.

Update the test body to correctly define the requisite iterator and
assert that the specific error created is the one thrown from the
invocation of `Promise.all`

Promise methods should not be enumerable, per https://people.mozilla.org/~jorendorff/es6-draft.html#table-4