Source/DestinationLightDom throw injection warnings

[rkirov]

To repro, do a pub serve in examples/

[Warning from InjectorGenerator on shadow-demo|web/main.dart with input angular|lib/core_dom/light_dom.dart]:
line 3, column 1 of package:angular/core_dom/light_dom.dart: SourceLightDom cannot be injected because it is an abstract type with no factory constructor.
@Injectable()
^^^^^^^^^^^^^
[Warning from InjectorGenerator on shadow-demo|web/main.dart with input angular|lib/core_dom/light_dom.dart]:
line 8, column 1 of package:angular/core_dom/light_dom.dart: DestinationLightDom cannot be injected because it is an abstract type with no factory constructor.
@Injectable()
^^^^^^^^^^^^^

[vsavkin]

Fixed by #1411