Use a reactElement.type prototype check instead of direct type equiva…

…lence

This allows DrawerNavigationChild and DrawerNavigationItem to be extended with a project's patterns.

example/components/DrawerNavigationExample.js

@@ -1,7 +1,6 @@
 import React, { Component } from 'react';
 import {
   StyleSheet,
-  View,
   Text,
   Image,
 } from 'react-native';
@@ -14,6 +13,14 @@ import {
 import { Ionicons } from '@exponent/vector-icons';
 import { Router } from '../main';
 
+class Heading extends DrawerNavigationChild {
+  renderDrawerItem() {
+    return (
+      <Text style={styles.headingText}>{this.props.title}</Text>
+    );
+  }
+}
+
 export default class DrawerNavigationExample extends Component {
 
   _renderHeader = () => {
@@ -69,9 +76,7 @@ export default class DrawerNavigationExample extends Component {
             initialRoute={Router.getRoute('home')}
           />
         </DrawerNavigationItem>
-        <DrawerNavigationChild>
-          <Text style={styles.headingText}>Meta</Text>
-        </DrawerNavigationChild>
+        <Heading title='Meta' />
         <DrawerNavigationItem
           id="another"
           selectedStyle={styles.selectedItemStyle}

src/drawer/ExNavigationDrawer.js

@@ -256,8 +256,8 @@ class ExNavigationDrawer extends PureComponent<any, Props, State> {
       }
 
       invariant(
-        child.type === ExNavigationDrawerItem || child.type === ExNavigationDrawerChild,
-        'All children of DrawerNavigation must be DrawerNavigationItems or DrawerNavigationChilds.',
+        child.type instanceof Object && ExNavigationDrawerChild.prototype.isPrototypeOf(child.type.prototype),
+        'All children of DrawerNavigation must be DrawerNavigationChild descendant components.'
       );
 
       return child;