title: GraphQL Type Error for Union Type when trying to run a query ([Error: Unknown type ...])
body: <p>I have set up a graphql schema where one of the fields ('items') is a list of objects which are a Union Type i.e. the list items can be one of two object types (in this case they are called 'packageOfferItem' and 'tileItem'). </p>
  
  <pre><code>var searchResultItemType = new graphql.GraphQLUnionType({
    name: 'SearchResultItems',
    types: [ packageOfferItem, tileItem ],
    resolveType: function (value) {
      if (value.type === 'packageOffer') {
        return packageOfferItem;
      } else if (value.type === 'tile') {
        return tileItem;
      } else {
        return null;
      }
    }
  });
  
  var searchResultItemsType = new graphql.GraphQLList(searchResultItemType);
  </code></pre>
  
  <p>The <code>items</code> field is then of type <code>searchResultItemsType</code>. </p>
  
  <p>In my tests I have a query which requests data from  <code>items</code> filed using the notation: </p>
  
  <pre><code>{ 
    items {
       ...on packageOfferItem {
         id
      }
    }
  }
  </code></pre>
  
  <p>In my test I get an error <code>[Error: Unknown type "packageOfferItem".] message: 'Unknown type "packageOfferItem".' } ]</code></p>
  
  <p>Where is this error coming from?? <code>packageOfferItem</code> is a graphql <code>NamedType</code> so I don't understand why there is an error in the query response. </p>
  
  <p>Introspection of the schema does not throw any errors so the schema itself is syntactically valid. </p>