Polymerのproperties、これinterfaceどう定義すればいいの

polymerのproperties.js

  properties = {
    prop1: String,
    prop2: {
      type: Number
    }
  };

試しに書いたけどtscがerror出すpolymer.d.ts

interface PolymerElement {
  properties?: {[propName:string]:BooleanConstructor|DateConstructor|NumberConstructor|StringConstructor|ArrayConstructor|PolymerElementProperty};
}
interface PolymerElementProperty {
  // Type constructor for the property
  type: BooleanConstructor|DateConstructor|NumberConstructor|StringConstructor|ArrayConstructor;
}