testUnregisterPlugin() {
    const editableField = new FieldConstructor('testField');
    const plugin = new TestPlugin();

    editableField.registerPlugin(plugin);
    editableField.unregisterPlugin(plugin);

    assertUndefined(
        'Unregistered plugin must not be in protected plugin map.',
        editableField.plugins_[plugin.getTrogClassId()]);

    editableField.dispose();
  }
Tests that calling unregisterPlugin will remove the plugin from
the map.