💥 src: Updated to latest lit-element and reinstall @material/mwc-button

Latest `lit-element` has fixed a known issue where missing types are not
exported thus breaks TS compilation for many users. To learn more about
the issue, please visit lit/lit-element#428.

`@material/mwc-button` has been updated to latest `lit-element` too.
Reinstalled to make the buttons work again!

package.json

@@ -42,7 +42,8 @@
     "watch": "npm run -- --watch"
   },
   "dependencies": {
-    "lit-element": "^2.0.0-rc.1",
+    "@material/mwc-button": "^0.3.5",
+    "lit-element": "^2.0.0-rc.2",
     "lit-html": "^1.0.0-rc.2"
   },
   "devDependencies": {

src/app-datepicker.ts

@@ -302,13 +302,13 @@ export class AppDatepicker extends LitElement {
   @property({ type: Number })
   public dragRatio: number = .15;
 
-  @property({ type: String })
-  // @ts-ignore
-  public startView: string = 'calendar';
+  // @property({ type: String })
+  // // @ts-ignore
+  // public startView: string = 'calendar';
 
-  @property({ type: String })
-  // @ts-ignore
-  public value: string;
+  // @property({ type: String })
+  // // @ts-ignore
+  // public value: string;
 
   @property({ type: String })
   private _startView: string = 'calendar';