WIP: add setNativeValue for change event

[kentcdodds]

What: This overrides the fireEvent.change function to one that allows users to set the target.value using a special setNativeValue function.

Why: (#152) Because React tracks when you set the value property on an input to keep track of the node's value. When you dispatch a change event, it checks it's last value against the current value and if they're the same it does not call any event handlers (as no change has taken place as far as react is concerned). So we have to set the value in a way that React's value setter function will not be called, which is where the setNativeValue comes into play.

How: Development of the setNativeValue function was a team effort: facebook/react#10135 (comment)

This is kinda cheating because it's overriding fireEvent.change. I think I'd rather we do this within dom-testing-library directly.

I'm also not super happy with the API:

fireEvent.change(input, {target: {value: 'hi'}})

It makes sense to me because in your onChange handler if you want the value you'll get it from event.target.value, but I worry that this API will make people think they can change other things about the target as well... I suppose we could make that work by Object.assigning all target properties onto the given node (while still treating the value property specially) 🤔

Another thing I've considered is just accepting a value option:

fireEvent.change(input, {value: 'hi'})

But I don't like that because the 2nd argument is supposed to be event init options... I'm open to feedback here.

Also, this PR is incomplete because we still have to handle select, mouseEnter, and mouseLeave. I'm not sure how to handle those. More investigation is needed.

I'm thinking that we should make these changes in dom-testing-library instead of here and we can address each of these one-by-one.

Checklist:

• Documentation
• Tests
• Ready to be merged
  
• Added myself to contributors table N/A
  

[codecov]

Codecov Report

Merging #153 into master will decrease coverage by 9.37%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage     100%   90.62%   -9.38%     
==========================================
  Files           1        1              
  Lines          19       32      +13     
  Branches        3        8       +5     
==========================================
+ Hits           19       29      +10     
- Misses          0        2       +2     
- Partials        0        1       +1

Impacted Files	Coverage Δ
src/index.js	90.62% <76.92%> (-9.38%)	⬇️

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[kentcdodds]

This is superseded by testing-library/dom-testing-library#85