[fixed] Issue reactjs#526 Import PropTypes as default import.

Reapplying changes from a rebase at 47d0d87.

src/components/ModalPortal.js

@@ -1,5 +1,5 @@
 import React, { Component } from "react";
-import { PropTypes } from "prop-types";
+import PropTypes from "prop-types";
 import * as focusManager from "../helpers/focusManager";
 import scopeTab from "../helpers/scopeTab";
 import * as ariaAppHider from "../helpers/ariaAppHider";