code: function getGradientDefs(doc) {
      var linearGradientEls = doc.getElementsByTagName('linearGradient'),
          radialGradientEls = doc.getElementsByTagName('radialGradient'),
          el,
          gradientDefs = { };
  
      for (var i = linearGradientEls.length; i--; ) {
        el = linearGradientEls[i];
        gradientDefs[el.getAttribute('id')] = el;
      }
  
      for (var i = radialGradientEls.length; i--; ) {
        el = radialGradientEls[i];
        gradientDefs[el.getAttribute('id')] = el;
      }
  
      return gradientDefs;
    }
docs: Parses an SVG document, returning all of the gradient declarations found in it
  @static
  @function
  @memberOf fabric
  @method getGradientDefs
  @param {SVGDocument} doc SVG document to parse
  @return {Object} Gradient definitions; key corresponds to element id, value -- to gradient definition element