Merge pull request #213 from Westbrook/master

Add disconnectedCallback stub.

Author: Steve Orvell

src/lib/updating-element.ts

@@ -396,6 +396,13 @@ export abstract class UpdatingElement extends HTMLElement {
     }
   }
 
+  /**
+   * Allows for `super.disconnectedCallback()` in extensions while
+   * reserving the possibility of making non-breaking feature additions
+   * when disconnecting at some point in the future.
+   */
+  disconnectedCallback() {}
+
   /**
    * Synchronizes property values when attributes change.
    */
@@ -610,4 +617,4 @@ export abstract class UpdatingElement extends HTMLElement {
    * * @param _changedProperties Map of changed properties with old values
    */
   protected firstUpdated(_changedProperties: PropertyValues) {}
-}
+}