code: function getFromAttribute(component, propertyName, source) {
    var value;
    var mappings = source.mappings || {};
    var route = component.name + '.' + propertyName;
    var primitiveAttribute = findAttribute(mappings, route);
    if (primitiveAttribute && source.hasAttribute(primitiveAttribute)) {
      value = source.getAttribute(primitiveAttribute);
    }
    return value;
  
    function findAttribute(mappings, route) {
      var attributes = Object.keys(mappings);
      for (var i = 0, l = attributes.length; i < l; i++) {
        var attribute = attributes[i];
        if (mappings[attribute] === route) {
          return attribute;
        }
      }
      return undefined;
    }
  }
docs: Gets the value for the component's property coming from a primitive
  attribute.
  
  Primitives have mappings from attributes to component's properties.
  The function looks for a present attribute in the source element which
  maps to the specified component's property.
  
  @param  {Component} component    Component to be found.
  @param  {string}    propertyName Component's property to be found.
  @param  {Element}   source       Element owning the component.
  @return {any}                    The value of the component's property coming
  from the primitive's attribute if any or
  `undefined`, otherwise.