[chore] use canUseDOM from exenv.

it's already implemented in this package.

Author: diasbruno

src/components/Modal.js

@@ -3,15 +3,13 @@ import ReactDOM from 'react-dom';
 import PropTypes from 'prop-types';
 import ModalPortal from './ModalPortal';
 import * as ariaAppHider from '../helpers/ariaAppHider';
-import SafeHTMLElement from '../helpers/safeHTMLElement';
+import SafeHTMLElement, {
+  canUseDOM
+} from '../helpers/safeHTMLElement';
 
 export const portalClassName = 'ReactModalPortal';
 export const bodyOpenClassName = 'ReactModal__Body--open';
 
-const canUseDOM = typeof window !== 'undefined' &&
-  typeof document !== 'undefined' &&
-  typeof document.createElement === 'function';
-
 const isReact16 = ReactDOM.createPortal !== undefined;
 const createPortal = isReact16 ?
   ReactDOM.createPortal :

src/helpers/safeHTMLElement.js

@@ -4,4 +4,6 @@ const EE = ExecutionEnvironment;
 
 const SafeHTMLElement = EE.canUseDOM ? window.HTMLElement : {};
 
+export const canUseDOM = EE.canUseDOM;
+
 export default SafeHTMLElement;