code: function getMixedValue(component, propertyName, source) {
    var value;
    var reversedMixins = source.mixinEls.reverse();
    for (var i = 0; value === undefined && i < reversedMixins.length; i++) {
      var mixin = reversedMixins[i];
      if (mixin.attributes.hasOwnProperty(component.name)) {
        if (!propertyName) {
          value = mixin.getAttribute(component.name);
        } else {
          value = mixin.getAttribute(component.name)[propertyName];
        }
      }
    }
    return value;
  }
docs: Gets the value for a component or component's property coming from mixins of
  an element.
  
  If the component or component's property is not provided by mixins, the
  functions will return `undefined`.
  
  @param {Component} component      Component to be found.
  @param {string}    [propertyName] If provided, component's property to be
  found.
  @param {Element}   source         Element owning the component.
  @return                           The value of the component or components'
  property coming from mixins of the source.