Polyfill Element for Node (#6607)

* Polyfill Element for Node

* Adds missing "const" keyword to Element (#6609)

Author: pshrmn

packages/react-router-dom/modules/Link.js

@@ -59,6 +59,8 @@ class Link extends React.Component {
 
 if (__DEV__) {
   const toType = PropTypes.oneOfType([PropTypes.string, PropTypes.object]);
+  // polyfill for Node
+  const Element = typeof Element === "undefined" ? function() {} : Element;
   const innerRefType = PropTypes.oneOfType([
     PropTypes.string,
     PropTypes.func,