webcomponents.js: Fix compatibility with lib.dom (#27796)

types/webcomponents.js/index.d.ts

@@ -36,7 +36,7 @@ export interface Polyfill {
 declare global {
     // This contains duplicates of some types in lib.dom.d.ts in order to support typescript 2.0
     interface ElementDefinitionOptions {
-        extends: string;
+        extends?: string;
     }
 
     interface ShadowRoot extends DocumentFragment {

types/webcomponents.js/tsconfig.json

@@ -6,8 +6,8 @@
             "dom"
         ],
         "noImplicitAny": true,
-        "noImplicitThis": false,
-        "strictNullChecks": false,
+        "noImplicitThis": true,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [

types/webcomponents.js/webcomponents.js-tests.ts

@@ -16,7 +16,7 @@ const XFoo = document.registerElement("x-foo", {
 
 const xFoo = new XFoo();
 xFoo.textContent = "";
-document.body.appendChild(xFoo);
+document.body!.appendChild(xFoo);
 
 window.CustomElements.hasNative;
 window.CustomElements.flags;