Prefer using inline prevals over exporting a constant

This will allow Rollup to optimize builds for UMD, ESM and CJS, without minification.

src/downshift.js

@@ -2,6 +2,7 @@
 
 import React, {Component} from 'react'
 import PropTypes from 'prop-types'
+import preval from 'preval.macro'
 import setA11yStatus from './set-a11y-status'
 import {
   cbToCb,
@@ -13,8 +14,6 @@ import {
   getA11yStatusMessage,
   unwrapArray,
   isDOMElement,
-  isPreact,
-  isReactNative,
   getElementProps,
   noop,
   requiredProp,
@@ -220,7 +219,7 @@ class Downshift extends Component {
 
   scrollHighlightedItemIntoView = () => {
     /* istanbul ignore else (react-native) */
-    if (!isReactNative) {
+    if (preval`module.exports = process.env.BUILD_REACT_NATIVE !== 'true'`) {
       const node = this.getItemNodeFromIndex(this.getState().highlightedIndex)
       const rootNode = this._rootNode
       scrollIntoView(node, rootNode)
@@ -541,7 +540,7 @@ class Downshift extends Component {
 
   getButtonProps = ({onClick, onKeyDown, ...rest} = {}) => {
     const {isOpen} = this.getState()
-    const enabledEventHandlers = isReactNative
+    const enabledEventHandlers = preval`module.exports = process.env.BUILD_REACT_NATIVE === 'true'`
       ? /* istanbul ignore next (react-native) */
         {
           onPress: composeEventHandlers(onClick, this.button_handleClick),
@@ -654,10 +653,12 @@ class Downshift extends Component {
 
   input_getOnChangeKey() {
     /* istanbul ignore next (preact) */
-    if (isPreact) {
+    if (preval`module.exports = process.env.BUILD_PREACT === 'true'`) {
       return 'onInput'
       /* istanbul ignore next (react-native) */
-    } else if (isReactNative) {
+    } else if (
+      preval`module.exports = process.env.BUILD_REACT_NATIVE === 'true'`
+    ) {
       return 'onChangeText'
     } else {
       return 'onChange'
@@ -674,7 +675,7 @@ class Downshift extends Component {
     this.internalSetState({
       type: Downshift.stateChangeTypes.changeInput,
       isOpen: true,
-      inputValue: isReactNative
+      inputValue: preval`module.exports = process.env.BUILD_REACT_NATIVE === 'true'`
         ? /* istanbul ignore next (react-native) */ event
         : event.target.value,
     })
@@ -708,7 +709,7 @@ class Downshift extends Component {
       this.items[index] = item
     }
 
-    const onSelectKey = isReactNative
+    const onSelectKey = preval`module.exports = process.env.BUILD_REACT_NATIVE === 'true'`
       ? /* istanbul ignore next (react-native) */ 'onPress'
       : 'onClick'
     return {
@@ -783,7 +784,7 @@ class Downshift extends Component {
     })
     this.previousResultCount = resultCount
     /* istanbul ignore else (react-native) */
-    if (!isReactNative) {
+    if (preval`module.exports = process.env.BUILD_REACT_NATIVE !== 'true'`) {
       setA11yStatus(status)
     }
   }, 200)
@@ -793,7 +794,7 @@ class Downshift extends Component {
     // and we don't want to update the status if the component has been umounted
     this._isMounted = true
     /* istanbul ignore if (react-native) */
-    if (isReactNative) {
+    if (preval`module.exports = process.env.BUILD_REACT_NATIVE === 'true'`) {
       this.cleanup = () => {
         this._isMounted = false
       }

src/utils.js

@@ -1,5 +1,3 @@
-import preval from 'preval.macro'
-
 let idCounter = 1
 
 /**
@@ -226,20 +224,6 @@ function isDOMElement(element) {
   }
 }
 
-/**
- * When built, this constant will signify if the current platform is React Native.
- * This can be used to optimize codepaths for other platforms without conflating
- * concepts because of the React Native target.
- * @return {Boolean} whether or not the platform is React Native
- */
-/* istanbul ignore next (react-native) */
-const isReactNative = preval`module.exports = process.env.BUILD_REACT_NATIVE === 'true'`
-
-// Disabling coverage here is necessary due to a weird deal with
-// babel-plugin-istanbul + preval.macro. No idea...
-/* istanbul ignore next (preact) */
-const isPreact = preval`module.exports = process.env.BUILD_PREACT === 'true'`
-
 /**
  * @param {Object} element (P)react element
  * @return {Object} the props
@@ -291,8 +275,6 @@ export {
   getA11yStatusMessage,
   unwrapArray,
   isDOMElement,
-  isPreact,
-  isReactNative,
   getElementProps,
   noop,
   requiredProp,