This reverts commit 561ee92.

We've had a lot of failed builds recently and it seems to coincide with
when we updated from Scala.js 0.6.8 to 0.6.11. This is an experiment to
see if reverting that change stabilizes the build.

This is a shot in the dark attempting to fix typelevel#1242.

The idea is that we can mark certain tests as slow and not run them when
running JS tests. Currently I've done this for all of the stack-safety
tests that I can find.

I don't really like this solution, but currently almost every PR is
resulting in a failed build, which is really problematic. One thing that
we could do is get rid of the `Platform.isJs` branch of the `slowTest`
implementation locally just before a release to make sure that these
tests all pass on JS. I think we only really run into these issues on
Travis CI.