Step 3: Add animation for sign in progress

It’s time to add animation! In this final step,
you’ll create the animation for the
LinearProgressIndicator at the top of the sign in
area. The animation has the following behavior:

When the app starts,
a tiny red bar appears across the top of the sign in area.

When one text field contains text,
the red bar turns orange and animates 0.15
of the way across the sign in area.

When two text fields contain text,
the orange bar turns yellow and animates half
of the way across the sign in area.

When all three text fields contain text,
the orange bar turns green and animates all the
way across the sign in area.
Also, the Sign up button becomes enabled.

Add an AnimatedProgressIndicator.
At the bottom of the file, add this widget:

    
class AnimatedProgressIndicator extends StatefulWidget {
  final double value;