Feature test directories (alternative to PR 5003)

[rdeforest]

This is an alternative to #5003 for making some tests not run (or even compile) when the runtime doesn't support a feature they rely on.

This change removes the need to edit Cakefile whenever tests are added for which rely on a feature not previously relied on. The tests need only be placed in a directory containing an index.coffee which tests for the feature they require.

The following pending PRs would benefit from this:

• Fix #4875: Asynchronous iterators #4893
• Lexer, Nodes: Use ES syntax for exponentiation ** and **= operators #4877
• Regular expressions s (dotAll) flag #4880
• Fix #4876: remove polyfill for object rest/spread #4884

[GeoffreyBooth]

I think I would prefer #5003. I think that over time we will increase the minimum version of Node required; Node 6 reaches end-of-life in April 2019, for example, and so that would seem to be the time to drop support for it (if not sooner) and the number of “exceptions” will simplify each time we do so. Also the folder-based approach lacks flexibility when it comes to extending the selective loading to the browser tests; presumably different files would be loaded for testing in browsers, or even testing in different browsers.

[GeoffreyBooth]

P.S. I don’t mean my closing this to be the final say on the matter. I’m happy to reopen if the other maintainers feel that this is the better of the two PRs. Just trying to de-clutter.