Add the /welcome route.
Create the connection to show the new screen.
In the build() method for SignUpApp,
add the following route below '/':

    
'/welcome': (context) => const WelcomeScreen(),

Run the app.
The Sign up button should now be enabled.
Click it to bring up the welcome screen.
Note how it animates in from the bottom.
You get that behavior for free.

Observations

The _showWelcomeScreen() function is used in the build()
method as a callback function. Callback functions are often
used in Dart code and, in this case, this means
“call this method when the button is pressed”.

The const keyword in front of the constructor is very
important. When Flutter encounters a constant widget, it
short-circuits most of the rebuilding work under the hood 
making the rendering more efficient.