add types for sync directive

Author: morbidick

index.d.ts

@@ -1 +1,2 @@
 export { LitNotify } from './notify';
+export { LitSync } from './sync';

sync.d.ts

@@ -0,0 +1,9 @@
+import { LitElement, UpdatingElement } from 'lit-element';
+
+type Constructor<T = LitElement> = new (...args: any[]) => T;
+
+declare class SyncElement {
+  sync(property: string, eventName?: string): void
+}
+
+export function LitSync<T extends UpdatingElement>(baseElement: Constructor<T>): T & SyncElement

sync.js

@@ -1,10 +1,12 @@
 import {directive} from "lit-html/lib/directive.js";
 import {eventNameForProperty} from "./notify.js";
 
+// eslint-disable-next-line valid-jsdoc
 /**
  * Mixin that provides a lit-html directive to sync a property to a child property
  *
- * @param {LitElement} baseElement - the LitElement to extend
+ * @template TBase
+ * @param {Constructor<TBase>} baseElement
  */
 export const LitSync = (baseElement) => class extends baseElement {
     constructor() {