use ReactDOM.createPortal if it is available

src/Portal.js

@@ -11,6 +11,9 @@ import ownerDocument from './utils/ownerDocument';
  * You can think of it as a declarative `appendChild()`, or jQuery's `$.fn.appendTo()`.
  * The children of `<Portal/>` component will be appended to the `container` specified.
  */
+
+const useCreatePortal = ReactDOM.createPortal !== undefined;
+
 class Portal extends React.Component {
 
   static displayName = 'Portal';
@@ -75,9 +78,13 @@ class Portal extends React.Component {
     // Save reference for future access.
     if (overlay !== null) {
       this._mountOverlayTarget();
-      this._overlayInstance = ReactDOM.unstable_renderSubtreeIntoContainer(
-        this, overlay, this._overlayTarget
-      );
+      if (useCreatePortal) {
+        this._overlayInstance = ReactDOM.createPortal(overlay, this._overlayTarget);
+      } else {
+        this._overlayInstance = ReactDOM.unstable_renderSubtreeIntoContainer(
+          this, overlay, this._overlayTarget
+        );
+      }
     } else {
       // Unrender if the component is null for transitions to null
       this._unrenderOverlay();