var mapVal = new TestUser();
InstanceMirror mirror = reflect(mapVal);
// getField() expects an instance of Symbol, not String
var futureValue = mirror.getField(new Symbol("age"));
// getField() returns an InstanceMirror, not a Future<InstanceMirror>
print("Field: age = ${futureValue.reflectee}");
Could it be that you updated your SDK, but not your Editor? The current Editor with the current SDK shows all of that as a warning, making troubleshooting this code rather trivial.