const aDialog = document.createElement('my-dialog') as NgElement & WithProperties‹{content: string}›;
aDialog.content = 'Hello, world!';
aDialog.content = 123;  // ‹-- ERROR: TypeScript knows this should be a string.
aDialog.body = 'News';  // ‹-- ERROR: TypeScript knows there is no `body` property on `aDialog`.