How do you define typings for custom elements?

@Component(...)
class MyContainer {
  @Input() message: string;
}

const container = document.createElement('my-container') as NgElement & WithProperties<{message: string}>;
container.message = 'Welcome to Angular elements!';
container.message = true;  // <-- ERROR: TypeScript knows this should be a string.
container.greet = 'News';  // <-- ERROR: TypeScript knows there is no `greet` property on `container`.