docs: 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;
  }