Add invariant for missing "to" property

[selbekk]

First off - thanks for creating this. Me, myself and my team - and thousands of other developers across the world - are building incredibly successful apps thanks to your hard work and sexy brains. I hugely appreciate your efforts.

Now, to business.

Whenever you forget to set the to property on a <Link /> component, You get the following error message:

If I scroll long enough up the error log, you'll see the error from prop types telling me I forgot to set the required to prop - but it usually takes me a minute to figure it out. With the rising of create-react-app - many people will probably not find it for a bit, because they are busy looking through the stack traces in the error overlay (like I was in my screen shot above).

This PR adds an invariant check (I'm very open to suggestions about the error message) to ensure it's sent in. If not, the code will throw before it crashes and burns deep down in the history package.

Thoughts? It's my first time contributing to your code base, so if I screwed up some formatting way of thinking, please accept my apologies.

[selbekk]

Oopsie, screwed up the test. Now we should be good to go 👍

[timdorr]

Makes sense. Thanks!