[ci-skip] Add comment explaining confusing check in _addPropertyToAtt…

…ributeMap

lib/mixins/properties-changed.js

@@ -134,6 +134,12 @@ export const PropertiesChanged = dedupingMixin(
       if (!this.hasOwnProperty('__dataAttributes')) {
         this.__dataAttributes = Object.assign({}, this.__dataAttributes);
       }
+      // This check is technically not correct; it's an optimization that
+      // assumes that if a _property_ name is already in the map (note this is
+      // an attr->property map), the property mapped directly to the attribute
+      // and it has already been mapped.  This would fail if
+      // `attributeNameForProperty` were overridden such that this was not the
+      // case.
       let attr = this.__dataAttributes[property];
       if (!attr) {
         attr = this.constructor.attributeNameForProperty(property);