code: public Map<String, JSType> getPropertyTypeMap() {
      ImmutableMap.Builder<String, JSType> propTypeMap = ImmutableMap.builder();
      for (String name : this.getPropertyNames()) {
        propTypeMap.put(name, this.getPropertyType(name));
      }
      return propTypeMap.build();
    }
docs: get the map of properties to types covered in an object type
  @return a Map that maps the property's name to the property's type