How to disable splash highlight of FlatButton in Flutter?

I have a FlatButton. I don't want the splash highlight when the button is clicked. I tried changing the splash colour to transparent, but that didn't work. Here is the code for my FlatButton.

Widget button = new Container(
  child: new Container(
    padding: new EdgeInsets.only(bottom: 20.0),
    alignment: Alignment.center,
    child: new FlatButton(
      onPressed: () {
        _onClickSignInButton();
      },
      splashColor: Colors.transparent,
      child: new Stack(
        alignment: Alignment.center,
        children: <Widget>[
          new Image.asset('images/1.0x/button1.png',
          ),
          new Text("SIGN IN",
            style: new TextStyle(
                color: Colors.white,
                fontWeight: FontWeight.bold,
                fontSize: 16.0
              ),
          )
        ],
      ),
    ),
  ),
);

I'd expect an invisible highlight color to do what you want:

new FlatButton({
  ...
  splashColor: Colors.transparent,  
  highlightColor: Colors.transparent, // makes highlight invisible too
})