get(src, crossOrigin, color) {
    const key = getCacheKey(src, crossOrigin, color);
    return key in this.cache_ ? this.cache_[key] : null;
  }
@param {string} src Src.
@param {?string} crossOrigin Cross origin.
@param {import("../color.js").Color|string|null} color Color.
@return {import("./IconImage.js").default} Icon image.