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

    const mockPlugin1 = new LooseMock(plugin);
    mockPlugin1.getTrogClassId().$returns('mockPlugin1');
    mockPlugin1.registerFieldObject(editableField);
    mockPlugin1.isEnabled(editableField).$anyTimes().$returns(true);
    mockPlugin1.isSupportedCommand('+indent').$returns(false);
    mockPlugin1.$replay();

    const mockPlugin2 = new LooseMock(plugin);
    mockPlugin2.getTrogClassId().$returns('mockPlugin2');
    mockPlugin2.registerFieldObject(editableField);
    mockPlugin2.isEnabled(editableField).$anyTimes().$returns(true);
    mockPlugin2.isSupportedCommand('+indent').$returns(true);
    mockPlugin2.execCommandInternal('+indent').$returns(true);
    mockPlugin2.execCommand('+indent')
        .$does(/**
                  @suppress {strictMissingProperties} suppression added to
                  enable type checking
                */
               () => {
                 mockPlugin2.execCommandInternal('+indent');
               });
    mockPlugin2.$replay();

    editableField.registerPlugin(mockPlugin1);
    editableField.registerPlugin(mockPlugin2);

    editableField.execCommand('+indent');

    mockPlugin1.$verify();
    mockPlugin2.$verify();
  }
Test that if the first plugin does not support execCommand, the other
plugins get a chance to handle the execCommand.
@suppress {missingProperties,strictMissingProperties} suppression added
to enable type checking