Fix failing test

[zpao]

If my name were test bot this would be an autogenerated task telling you that tests are failing after 32423a8 - https://travis-ci.org/facebook/react/builds/8680688

That's you @petehunt!

[petehunt]

OK, the tests passed internally but not in open-source because dumpCache() is a no-op, so double DOM property injection is happening and throwing.

[benjamn]

I have ideas.

If we take the plunge of forking browserify, then a lot of things become possible. A proper mocking system would be relatively easy to implement if we could redefine browserify's implementation of require.

But I don't think we have to fork browserify. Instead, it looks like we can transform our own source files during testing so that require("mock-modules") can manipulate their module.exports, swapping a mocked version of exports in and out as necessary. We can do this either in bin/jsx or using a browserify source transform. I prefer bin/jsx because then, in principle at least, mocking would work with any module packaging tool (i.e. it would not depend on the browserify interface).

Need to investigate further, but I think this is really promising.