As an added bonus, the jasmine web interface now groups tests by file.
Test Plan:
grunt test passes on b2507066, the parent of 566f8b2e (which committed a workaround for buggy module mocking).
This was not necessary when we were running each test in its own
`<iframe>`, and it doesn't seem to affect any test behavior currently, but
it seems wise for the sake of test isolation and hygiene.
We don't sync upstream polyfills (because we don't have a story for how
they would be used), so this needs to be updated manually.
Sacrificed some negligible performance optimizations to reduce the number
of different cases from four to one.
It's important to test this implementation in PhantomJS, since that's the
only browser that I know of where built-in functions sometimes do not have
a `.prototype`.
This cuts the running time of `grunt phantom:run` from 4.4s to 3.1s on my
machine, because we no longer have to load/execute a separate instance of
`react-test.js` in a separate `<iframe>` for each test.
