T extends Object> class
An Expando allows adding new properties to objects.
Does not work on numbers, strings, booleans,
dart:ffi structs, or
Expando does not hold on to the added property value after an object
Since you can always create a new number that is identical to an existing
number, it means that an expando property on a number could never be
released. To avoid this, expando properties cannot be added to numbers.
The same argument applies to strings, booleans and
null, which also have
literals that evaluate to identical values when they occur more than once.
There is no restriction on other classes, even for compile time constant objects. Be careful if adding expando properties to compile time constants, since they will stay alive forever.