title: Invariant type error when using Jest mount
body: <p>So I have a React Class lets call it A. I'm running a jest test on it, but I keep getting an </p>
  
  <p><code>Invariant Violation: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: object.</code></p>
  
  <p>In the React Class I'm doing : 
  <code>export class A extends Component ..</code></p>
  
  <p>In the jest test file I'm doing :
  <code>import { A } from './A.js'</code></p>
  
  <p>when I run :
  <code>const wrapper = mount(&lt;A /&gt;)</code></p>
  
  <p>I get the error above. I'm running on a jsdom environment for jest. I'm kind of at a loss why this wouldn't work. I've read that some people export as default, but I don't see why doing a proper name import shouldn't work. Does anyone have any ideas what could be the issue?</p>
  
  <p>real code : 
  jest file :</p>
  
  <pre><code>import Adapter from 'enzyme-adapter-react-16';
  import Enzyme, { shallow, mount } from 'enzyme';
  import React from 'react';
  
  import { A } from '../A';
  
  Enzyme.configure({ adapter: new Adapter() });
  
  /**
   * Test Suite
   */
  describe('A test', () =&gt; {
   it('calls componentDidMount', () =&gt; {
     const wrapper = mount(&lt;A /&gt;);
   })
  })
  </code></pre>
  
  <p>react class : </p>
  
  <pre><code>import React, { Component } from 'react';
  import PropTypes from 'prop-types';
  
  export class A extends Component {
  ...
  }
  </code></pre>
  
  <p>jest config : </p>
  
  <pre><code>module.exports = {
      clearMocks: true,
  
      // The directory where Jest should output its coverage files
      coverageDirectory: 'coverage',
  
      // The test environment that will be used for testing
      testEnvironment: 'jsdom',
      testURL: 'http://localhost/',
  
      // Directory to search for tests
      roots: ['src/'],
  
      // The glob patterns Jest uses to detect test files
      testMatch: [
          '**/__tests__/**/*.[jt]s?(x)',
          '**/?(*.)+(spec|test).[tj]s?(x)'
      ],
  
      // An array of regexp pattern strings that are matched against all test paths, matched tests are skipped
      testPathIgnorePatterns: [
          '/node_modules/'
      ],
  
      snapshotSerializers: [
          'enzyme-to-json/serializer'
      ]
  };
  
  </code></pre>