Add resource leak detection in test suite

[kostaleonard]

I am new to Flutter, so this may not be an actual issue. I am concerned about potential memory (or other resource) leaks when, for example, a developer fails to call dispose() on objects referenced in a State object during the latter's disposal (see below). Can we add unit tests or a tool in CI/CD that will check for leaks?

See this flutter devtools issue for more info.

@override
  void dispose() {
    //Did I remember to dispose() everything?
    _textEditingController.dispose();
    _textFieldFocusNode.dispose();
    super.dispose();
  }