Change export of "is-dev" to be a constant to improve code compilation

src/components/node.js

@@ -9,7 +9,7 @@ import Leaf from './leaf'
 import Void from './void'
 import scrollTo from '../utils/scroll-to'
 import warning from '../utils/warning'
-import isDev from '../utils/is-dev'
+import IS_DEV from '../utils/is-dev'
 
 /**
  * Debug.
@@ -101,7 +101,7 @@ class Node extends React.Component {
 
     // If the node has changed, update.
     if (nextProps.node != this.props.node) {
-      if (!isDev() || !Immutable.is(nextProps.node, this.props.node)) {
+      if (!IS_DEV || !Immutable.is(nextProps.node, this.props.node)) {
         return true
       } else {
         warning('Encountered different references for identical node values in "shouldComponentUpdate". Check that you are preserving references for the following node:\n', nextProps.node)

src/utils/is-dev.js

@@ -1,4 +1,4 @@
-const __DEV__ = (
+const IS_DEV = (
   typeof process !== 'undefined' &&
   process.env &&
   process.env.NODE_ENV !== 'production'
@@ -6,9 +6,7 @@ const __DEV__ = (
 
 /**
  * Return true if running slate in development
- * @return {Boolean} dev
+ * @type {Boolean}
  */
 
-export default function isDev() {
-  return __DEV__
-}
+export default IS_DEV

src/utils/warning.js

@@ -1,4 +1,4 @@
-import isDev from './is-dev'
+import IS_DEV from './is-dev'
 
 /**
  * Log a development warning.
@@ -6,7 +6,7 @@ import isDev from './is-dev'
  */
 
 export default function warning(message, ...more) {
-  if (!isDev()) {
+  if (!IS_DEV) {
     return
   }