testPluginExecCommand() {
    const plugin = new TestPlugin();
    let passedCommand;
    let passedArg;
    plugin.execCommand = (command, arg) => {
      passedCommand = command;
      passedArg = arg;
    };

    const editableField = new FieldConstructor('testField');
    editableField.registerPlugin(plugin);
    plugin.enable(editableField);
    plugin.isSupportedCommand = functions.constant(true);

    editableField.execCommand('+indent', true);
    // Verify that the plugin's execCommand was called with the correct
    // args.
    assertEquals('+indent', passedCommand);
    assertTrue(passedArg);

    passedCommand = null;
    passedArg = null;
    plugin.isSupportedCommand = functions.constant(false);

    editableField.execCommand('+outdent', false);
    // Verify that a plugin's execCommand is not called if it isn't a
    // supported command.
    assertNull(passedCommand);
    assertNull(passedArg);

    editableField.dispose();
    plugin.dispose();
  }
Test that if a plugin has an execCommand function, it gets called
but only for supported commands.