Give dom.Value a more specific constructor

[zslayton]

Typescript requires that mixins have a single constructor which accepts a single spread parameter.

constructor(...args: any[]) {
  // ...
}

This means that even though all classes extending the dom.Value mixin are required to provide _ionAnnotations: string[] at construction time, we cannot statically enforce that it be passed in. We currently rely on subclasses calling this._setAnnotations in their constructor after invoking super(). If this requirement is ever relaxed, we should take advantage of it.