多亏了DART的factory constructors，很容易构建一个Singleton:
class Singleton {
static final Singleton _singleton = Singleton._internal();
factory Singleton() {
return _singleton;
}
Singleton._internal();
}
你可以这样建造它
main() {
var s1 = Singleton();
var s2 = Singleton();
print(identical(s1, s2)); // true
print(s1 == s2); // true
}