fix(preact): fix umd export for preact

[razh]

What: Use preact as the global for the UMD preact bundle instead of react.

Why:

The UMD build of preact references a global.React:

// umd
(function (global, factory) {
	typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory(require('preact')) :
	typeof define === 'function' && define.amd ? define(['preact'], factory) :
	(global.Downshift = factory(global.React));
}(this, (function (React) { 'use strict';

/* ... */

})));

https://unpkg.com/downshift@1.3.0/dist/downshift.preact.umd.js

When targeting preact, rollup-plugin-alias changes all react module references to preact:

// cjs
var React = require('preact');

preact isn't provided as a global to rollup, so for the UMD build, rollup guesses React based off of the variable name.

Before

import React from 'react'
/*                   │
                     │
            rollup-plugin-alias
                     │
                     ▼                    */
import React from 'preact'
/*       │
         │
         └── rollup guesses from ───┐
                 variable name      │
                                    ▼     */
global.Downshift = factory(global.React)

After

import React from 'react'
/*                   │
                     │
            rollup-plugin-alias
                     │
                     ▼                    */
import React from 'preact'
/*                   │
              rollup looks up
               in `globals`
                     └──────────────┐
                                    ▼     */
global.Downshift = factory(global.preact)

How:

Declare preact as a global instead.

Checklist:

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

[kentcdodds]

Thanks!

[codecov-io]

Codecov Report

Merging #169 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #169   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         267    267           
  Branches       64     64           
=====================================
  Hits          267    267

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2e8d0ae...1618f45. Read the comment docs.