Look at the code file microtest.dart, where microtest is an internal package as seen in the previous recipe:

import 'package:microtest/microtest.dart';

void main() {
 Person p1 = new Person("Jim Greenfield", 178, 86.0);
 print('${p1.name} weighs ${p1.weight}' );
 // lots of other code and method calls
 // p1 = null;
 // working again with p1:
 assert(p1 is Person); 
 p1.weight = 100.0;
 print('${p1.name} now weighs ${p1.weight}' );
} 

We import the microtest library, which contains the definition of the Person class. In main(), we create a Person object p1, go through lots of code, and then want to work with p1 again, possibly in a different method of another class. How do we know that p1 still references a Person object? In the previous snippet...