Don't override the goog namespace if it already exists

lib/utils/boot.html

@@ -39,13 +39,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
   When using Closure Compiler, goog.reflect.objectProperty(property, object) is replaced by the munged name for object[property]
   We cannot alias this function, so we have to use a small shim that has the same behavior when not compiling.
   */
-  window.goog = {
-    reflect: {
-      objectProperty(s, o) {
-        return s;
-      }
+  window.goog = window.goog || {};
+  window.goog.reflect = window.goog.reflect || {
+    objectProperty(s, o) {
+      return s;
     }
-  }
+  };
   /* eslint-enable */
 
 })();