code: function buildSpriteString(icons) {
    const symbols = Object.keys(icons)
      .map(icon => toSvgSymbol(icon, icons[icon]))
      .join('');
  
    return `<svg xmlns="${DEFAULT_ATTRS.xmlns}"><defs>${symbols}</defs></svg>`;
  }
docs: Build an SVG sprite string containing SVG symbols.
  @param {Object} icons
  @returns {string}