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

    assertNull(
        'Must not be able to get unregistered plugins by class id.',
        editableField.getPluginByClassId(plugin.getTrogClassId()));

    editableField.registerPlugin(plugin);
    assertEquals(
        'Must be able to get registered plugins by class id.', plugin,
        editableField.getPluginByClassId(plugin.getTrogClassId()));
    editableField.dispose();
  }
Tests that registered plugins can be fetched by their id.