resolveIcon(namespace, iconset, icon) {
    const resolver = this.__iconResolvers.get(namespace);
    if (resolver) {
      return resolver(iconset, icon);
    }
    throw new Error(`Could not find any icon resolver for namespace ${namespace}.`);
  }
Resolves icon for the given parameters. Returns the icon as a svg string.

@param {string} namespace
@param {string} iconset
@param {string} icon
@returns {Promise<TemplateResult>}