Add getProperties method

packages/app-shell/src/app-shell.js

@@ -225,7 +225,7 @@ export class AppShell extends ScopedElementsMixin(AdapterLitElement) {
     }
 
     static get properties() {
-        return Object.assign({
+        return this.getProperties({
             lang: { type: String, reflect: true },
             src: { type: String },
             basePath: { type: String, attribute: 'base-path' },
@@ -243,7 +243,7 @@ export class AppShell extends ScopedElementsMixin(AdapterLitElement) {
             shellName: { type: String, attribute: "shell-name" },
             shellSubname: { type: String, attribute: "shell-subname" },
             noBrand: { type: Boolean, attribute: "no-brand" }
-        }, super.properties);
+        });
     }
 
     _updateAuth(login) {

packages/provider/src/adapter-lit-element.js

@@ -86,6 +86,10 @@ export class AdapterLitElement extends LitElement {
         };
     }
 
+    static getProperties(properties = {}) {
+        return Object.assign(properties, super.properties);
+    }
+
     attributeChangedCallback(name, oldValue, newValue) {
         switch(name) {
             case 'subscribe':