title: Updating state with immutability helper update
body: <p>I want to update my state with the immutability-helper function <em>update</em> (<a href="https://github.com/kolodny/immutability-helper" rel="nofollow noreferrer">https://github.com/kolodny/immutability-helper</a>)</p>
  
  <pre><code>state = {
      field: '',
      items: ['Item 1', 'Item 2', 'Item 3', 'Item 4', 'Item 5', 'Item 6']
    };
  
  addElementToList() {
   const i = this.state.items.length;
   const newState = ()=&gt; {
    update(this.state,
      {
        items: {
          $push: [
            {
              [i]: this.state.field
            }
          ]
        }
       }
     )
   };
   this.setState = newState;
  }
  </code></pre>
  
  <p>Unfortunately, I get the following error when the function is executed.</p>
  
  <pre><code>Uncaught TypeError: (0 , _immutabilityHelper.update) is not a function
  </code></pre>
  
  <p>What have I missed,any thoughts about this?</p>