Export UpdatingElement as ReactiveElement (#1177)

Author: justinfagnani

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### 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).
+* Adds an export of `UpdatingElement` as `ReactiveElement` for forward-compatibility with lit-element 3.0 [#1177](https://github.com/Polymer/lit-element/issues/1177).
 
 ### Fixed
 * Fixes an issue with `queryAssignedNodes` when applying a selector on a slot that included text nodes on older browsers not supporting Element.matches [#1088](https://github.com/Polymer/lit-element/issues/1088).

src/lit-element.ts

@@ -59,6 +59,7 @@ import {render, ShadyRenderOptions} from 'lit-html/lib/shady-render.js';
 import {PropertyValues, UpdatingElement} from './lib/updating-element.js';
 
 export * from './lib/updating-element.js';
+export {UpdatingElement as ReactiveElement} from './lib/updating-element.js';
 export * from './lib/decorators.js';
 export {html, svg, TemplateResult, SVGTemplateResult} from 'lit-html/lit-html.js';
 import {supportsAdoptingStyleSheets, CSSResult, unsafeCSS} from './lib/css-tag.js';