Provide access to a type from a `TypeMirror`

[eernstg]

A TypeMirror has a member hasReflectedType and a member reflectedType, and the latter returns a Type for the type which is mirrored by this TypeMirror.

Here is an example of the kind of data provided by reflectedType:

      <Type>[
        prefix0.A,
        const r.FakeType(r'test_reflectable.test.class_property_test._B'),
        prefix0.D,
        prefix0.C
      ]

In many cases, these Type objects are obtained by evaluating a typename denoting a class or type alias (for instance, the library imported with prefix prefix0 above contains class A, class C, class D).

We could actually create a construct that would enable the use of such types as types:

    <void Function(void Function<X>())>[
      (f) => f<prefix0.A>(),
      (f) => throw "Cannot `callWithReflectedType` when type is private",
      (f) => f<prefix0.D>(),
      (f) => f<prefix0.C>(),
    ];

The TypeMirror class would have a new member callWithReflectedType:

abstract class TypeMirror implements DeclarationMirror {
  ...
  void callWithReflectedType(void Function<X>() f) =>
    reflector.typeCallers[reflectedTypeIndex](f);
  ...
}

We would then be able to use it as follows:

@reflector
class C {}

void main() {
  TypeMirror typeMirror = reflector.reflectType(C);
  late Map map;
  if (typeMirror.hasReflectedType) { // This will be true, so we ignore the `else` case.
    typeMirror.callWithReflectedType(<X>() => map = <String, X>{});
  }
}

The callback which is passed to callWithReflectedType will receive the actual type mirrored by the TypeMirror as a normal Dart type argument, which means that we can use that type under the name X (or whatever we choose to call the type variable of the callback). For instance, we can create a Map<String, C> when the type mirror is a mirror of C.