Flutter: how to make a TextField with HintText but no Underline?

Question

This is what I'm trying to make:

In the Flutter docs for Text Fields (https://flutter.io/text-input/) it says you can remove the underline by passing null to the decoration. However, that also gets rid of the hint text.

I do not want any underline whether the text field is focused or not.

UPDATE: updated accepted answer to reflect changes in Flutter SDK as of April 2020.

Answer 1

Do it like this:

TextField(
  decoration: new InputDecoration.collapsed(
    hintText: 'Username'
  ),
),

or if you need other stuff like icon, set the border with InputBorder.none

InputDecoration(
    border: InputBorder.none,
    hintText: 'Username',
  ),
),