$ npm run compile
[4:44:16 PM] File change detected. Starting incremental compilation...
demo-children-extended.ts:5:17 - error TS2339: Property 'children' does not exist on type 'DemoChildren'.
5 return this.children
~~~~~~~~
[4:44:17 PM] Found 1 error. Watching for file changes.
However, note the compiler doesn't warn you on demo-children-lit-element.ts which has exactly the same code except LitElement is imported from lit-element instead of lit-element, WHY?