title: I get a contextTypes error when I try to test a very simple component
body: <p>I am new to reactjs and trying to write a unit test for a simple function. 
  I am using enzyme and here is my test:</p>
  
  <pre><code>import React from 'react';
  import { shallow } from 'enzyme';
  import {P} from "../../src/app/components/T";
  import ToDoItem from "../../src/app/components/ToDoItem";
  
  function mockItem(overrides) {
    // … create mock ToDo Item
  }
  
  describe('&lt;P /&gt;', () =&gt; {
    it('renders without exploding', () =&gt; {
      const wrapper = shallow(&lt;P attach={ true } /&gt;);
      expect(wrapper.length).toEqual(1);
    });
  });
  </code></pre>
  
  <p>and here is my component:</p>
  
  <pre><code>import  React from "react";
  
  export class T extends  React.Component{
    render() {
      return (
      &lt;div&gt;
          &lt;div className="row panel"&gt;
              &lt;div className="col-sm-12 header"&gt;uuuuu&lt;/div&gt;
          &lt;/div&gt;
      &lt;/div&gt;
      );
    }
  }
  </code></pre>
  
  <p>But when I run it I get the following error:</p>
  
  <pre><code>&lt;P /&gt; renders without exploding:
   TypeError: Cannot read property 'contextTypes' of undefined
    at ShallowComponentWrapper._maskContext (node_modules\react-dom\lib\ReactCompositeComponent.js:461:33)
    at ShallowComponentWrapper._processContext (node_modules\react-dom\lib\ReactCompositeComponent.js:481:30)
    at ShallowComponentWrapper.mountComponent (node_modules\react-dom\lib\ReactCompositeComponent.js:180:30)
    at Object.mountComponent (node_modules\react-dom\lib\ReactReconciler.js:46:35)
    at ReactShallowRenderer._render (node_modules\react-dom\lib\ReactShallowRenderer.js:126:23)
    at _batchedRender (node_modules\react-dom\lib\ReactShallowRenderer.js:79:12)
    at Object.batchedUpdates (node_modules\react-dom\lib\ReactDefaultBatchingStrategy.js:60:14)
    at Object.batchedUpdates (node_modules\react-dom\lib\ReactUpdates.js:97:27)
    at ReactShallowRenderer.render (node_modules\react-dom\lib\ReactShallowRenderer.js:106:18)
    at ReactShallowRenderer.render (node_modules\enzyme\build\react-compat.js:158:39)
    at node_modules\enzyme\build\ShallowWrapper.js:90:26
    at ReactDefaultBatchingStrategyTransaction.perform (node_modules\react-dom\lib\Transaction.js:140:20)
    at Object.batchedUpdates (node_modules\react-dom\lib\ReactDefaultBatchingStrategy.js:62:26)
    at batchedUpdates (node_modules\react-dom\lib\ReactUpdates.js:97:27)
    at node_modules\enzyme\build\ShallowWrapper.js:89:41
    at withSetStateAllowed (node_modules\enzyme\build\Utils.js:224:3)
    at new ShallowWrapper (node_modules\enzyme\build\ShallowWrapper.js:88:38)
    at shallow (node_modules\enzyme\build\shallow.js:19:10)
    at Context.&lt;anonymous&gt; (C:/tj/reactjs-basics/test/components/test.js:16:25)
  </code></pre>
  
  <p>I do not know if I am missing any library or if I am doing something wring in my code. Can anyone help?</p>