Modified the todo example.

Instead of only using the index to supress the warning it now uses the
text + index to also ensure object iteration order.

_js/examples/todo.js

@@ -2,7 +2,7 @@ var TODO_COMPONENT = `
 var TodoList = React.createClass({
   render: function() {
     var createItem = function(itemText, index) {
-      return <li key={index}>{itemText}</li>;
+      return <li key={index + itemText}>{itemText}</li>;
     };
     return <ul>{this.props.items.map(createItem)}</ul>;
   }