code: function defineOneWayBinding (key, handler) {
      var attr = $attrs.$normalize('md-' + key);
      if (handler) defineProperty(key, handler);
      $attrs.$observe(attr, function (newValue) { ctrl[ key ] = newValue; });
    }
docs: Creates a one-way binding manually rather than relying on AngularJS's isolated scope
  @param key
  @param handler