[1.0] Fix gatsby-link for undefined nextState

[0x80]

• Solves errors in production when /route/child/ is available but /route/ is not.

@KyleAMathews I thought fetching routes like this is actually a good example of where you'd want to be have a debug statement in place to just switch on. I think the size of the debug library is negligible, but if you disagree I can take it out of course.

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[KyleAMathews]

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[KyleAMathews]

Yeah... don't think shipping code that 99.999% of page loads won't use makes sense.

If we won't browser logging, why don't we just add a build flag or something equivalent? So gatsby build --debug-logging or whatever. Then that sets a environment variable in webpack and in the code we have if (process.env.DEBUG) { console.log('something') }. This code would be removed for normal builds and be left in when you want to debug something.

Also the debug code would always run during development.

We could use console.group to make things fairly tidy.

[0x80]

@KyleAMathews ok fair enough. I'll give it some more thought later and strip the debugging for now

[KyleAMathews]

Awesome, thanks!