Replace disconnectedCallback stub since this change is breaking.

lib/mixins/properties-mixin.html

@@ -200,6 +200,16 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         this._enableProperties();
       }
 
+      /**
+       * Called when the element is removed from a document
+       * @suppress {missingProperties} Super may or may not implement the callback
+       */
+      disconnectedCallback() {
+        if (super.disconnectedCallback) {
+          super.disconnectedCallback();
+        }
+      }
+
     }
 
     return PropertiesMixin;