Update Advanced Types.md as per received comment.

Author: gburdeti

pages/Advanced Types.md

@@ -253,11 +253,11 @@ Here's a simple mixin example:
 function extend<T, U>(first: T, second: U): T & U {
     let result = <T & U>{};
     for (let id in first) {
-        result[id] = first[id];
+        (<any>result)[id] = (<any>first)[id];
     }
     for (let id in second) {
         if (!result.hasOwnProperty(id)) {
-            result[id] = second[id];
+            (<any>result)[id] = (<any>second)[id];
         }
     }
     return result;