constructor(xmlDeclaration, doctypeDeclaration, rootAttributes, addSVGNamespaces, transform) {
    this.DEFAULT_SVG_NAMESPACE = 'http://www.w3.org/2000/svg';
    this.XLINK_NAMESPACE = 'http://www.w3.org/1999/xlink';

    this.xmlDeclaration = xmlDeclaration || '';
    this.doctypeDeclaration = doctypeDeclaration || '';
    this.rootAttributes = { ...rootAttributes };
    this.transform = transform;
    this.content = [];
    this._serialized = null;

    if (addSVGNamespaces) {
      this.rootAttributes.xmlns = this.DEFAULT_SVG_NAMESPACE;
      this.rootAttributes['xmlns:xlink'] = this.XLINK_NAMESPACE;
    }
  }
SVGSprite

@param {string} xmlDeclaration       XML declaration
@param {string} doctypeDeclaration   Doctype declaration
@param {object} rootAttributes       Root attributes
@param {boolean} addSVGNamespaces    Add default SVG namespaces
@param {Array} transform             List of post-processing transform callbacks