standardProperty = (options, element) => {
    // When decorating an accessor, pass it through and add property metadata.
    // Note, the `hasOwnProperty` check in `createProperty` ensures we don't
    // stomp over the user's accessor.
    if (element.kind === 'method' && element.descriptor &&
        !('value' in element.descriptor)) {
        return Object.assign(Object.assign({}, element), { finisher(clazz) {
                clazz.createProperty(element.key, options);
            } });
    }
    else {
        // createProperty() takes care of defining the property, but we still
        // must return some kind of descriptor, so return a descriptor for an
        // unused prototype field. The finisher calls createProperty().
        return {
            kind: 'field',
            key: Symbol(),
            placement: 'own',
            descriptor: {},
            // When @babel/plugin-proposal-decorators implements initializers,
            // do this instead of the initializer below. See:
            // https://github.com/babel/babel/issues/9260 extras: [
            //   {
            //     kind: 'initializer',
            //     placement: 'own',
            //     initializer: descriptor.initializer,
            //   }
            // ],
            initializer() {
                if (typeof element.initializer === 'function') {
                    this[element.key] = element.initializer.call(this);
                }
            },
            finisher(clazz) {
                clazz.createProperty(element.key, options);
            }
        };
    }
}
Class decorator factory that defines the decorated class as a custom element.

```
@customElement('my-element')
class MyElement {
  render() {
    return html``;
  }
}
```

@param tagName The name of the custom element to define.