Add getUpdateComplete() for forward-compat with LitElement 3.0

Author: justinfagnani

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 * Added the `@state()` decorator as an alias for `@internalProperty()`, and deprecated `@internalProperty()` which will be renamed to `@state` in lit-element 3.0. [#1162](https://github.com/Polymer/lit-element/issues/1162).
+* Added `UpdatingElement.prototype.getUpdateComplete()` and deprecated `_getUpdateComplete()` for forward compibility with lit-element 3.0.
 
 ### Added
 * Adds a `static shadowRootOptions` property for specifying shadow root options. This is a slightly simpler alternative to implementing a custom `createRenderRoot` method [#1147](https://github.com/Polymer/lit-element/issues/1147).

src/lib/updating-element.ts

@@ -832,8 +832,30 @@ export abstract class UpdatingElement extends HTMLElement {
    *       await this._myChild.updateComplete;
    *     }
    *   }
+   * @deprecated Override `getUpdateComplete()` instead for forward
+   *     compatibility with `lit-element` 3.0 / `@lit/reactive-element`.
    */
   protected _getUpdateComplete() {
+    return this.getUpdateComplete();
+  }
+
+  /**
+   * Override point for the `updateComplete` promise.
+   *
+   * It is not safe to override the `updateComplete` getter directly due to a
+   * limitation in TypeScript which means it is not possible to call a
+   * superclass getter (e.g. `super.updateComplete.then(...)`) when the target
+   * language is ES5 (https://github.com/microsoft/TypeScript/issues/338).
+   * This method should be overridden instead. For example:
+   *
+   *   class MyElement extends LitElement {
+   *     async getUpdateComplete() {
+   *       await super.getUpdateComplete();
+   *       await this._myChild.updateComplete;
+   *     }
+   *   }
+   */
+  protected getUpdateComplete() {
     return this._updatePromise;
   }