missing error reporting when @WidgetbookTheme is used on properties

[jenshor]

Description
Using @WidgetbookTheme on a ThemeData defining property results in incorrectly generated code.

Example code:
@WidgetbookTheme(name: 'Something')
ThemeData get myTheme => ThemeData.dark();

Leads to generated code:

Widgetbook.material(
      appInfo: AppInfo(
        name: 'Meal App',
      ),
      supportedLocales: locales,
      localizationsDelegates: delegates,
      themes: [
        WidgetbookTheme(
          name: 'Dark',
          data: getDarkThemeData(),
        ),
        WidgetbookTheme(
          name: 'Light',
          data: getLightThemeData(),
        ),
        WidgetbookTheme(
          name: 'Something',
          data: myTheme(),
        ),
      ],
... 

Expected Behavior
Generated code should be

WidgetbookTheme(
  name: 'Something',
  data: myTheme,
),