Define the typing indicator widget

The typing indicator exists within its own widget so that
it can be used anywhere in your app. As with any widget
that controls animations, the typing indicator needs to
be a stateful widget. The widget accepts a boolean value 
that determines whether the indicator is visible.
This speech-bubble-typing indicator accepts a color
for the bubbles and two colors for the light and dark
phases of the flashing circles within the large speech bubble.

Define a new stateful widget called TypingIndicator.

Make room for the typing indicator

The typing indicator doesn’t occupy any space when it
isn’t displayed. Therefore, the indicator needs to grow
in height when it appears, and shrink in height
when it disappears.