Support useExisting vs useClass

[leonadler]

In Angular, there is a difference between these two type of providers.

This will create 2 different instances of DependencyA:

const providers = [
  DependencyA,
  { provide: DependencyB, useClass: DependencyA }
];

While this will create 1 instance of DependencyA and provide it for both tokens:

const providers = [
  DependencyA,
  { provide: DependencyB, useExisting: DependencyA }
];