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Doesn't this include the new class syntax? If so, will the standard libraries change as well to use the new classes?

Yes, classes are stable in V8 4.5 so Node.js 4.0 will have classes without runtime flags.

Is there a list with all the supported ES2015 features?

Probably the same as io.js[0].

Why are these lists always missing import/export/modules?

Because Node uses CommonJS modules.