addIconResolver(namespace, iconResolver) {
    if (this.__iconResolvers.has(namespace)) {
      throw new Error(`An icon resolver has already been registered for namespace: ${namespace}`);
    }
    this.__iconResolvers.set(namespace, iconResolver);
  }
Adds an icon resolver for the given namespace. An icon resolver is a
function which takes an icon set and an icon name and returns an svg
icon as a TemplateResult. This function can be sync or async.

@param {string} namespace
@param {(iconset: string, icon: string) => TemplateResult | Promise<TemplateResult> | nothing | Promise<nothing> } iconResolver