Dependency injection example for macros (#3015)

This adds a basic Dagger2-like DI framework example, loosely based on this youtube video https://www.youtube.com/watch?v=oK_XtfXPkqw. There are 3 macros, @Injectable (for injectable classes), @provides (for modules, it is how they provide specific implementations), and @component which takes a bunch of modules and uses them to provide the final instances of things.

The interesting stuff is in working/macros/example/lib/injectable.dart :).

Author: jakemac53

analysis_options.yaml

@@ -5,6 +5,7 @@ analyzer:
     # TODO: remove these when the analyzer supports macros
     - working/macros/example/lib/auto_dispose.dart
     - working/macros/example/lib/data_class.dart
+    - working/macros/example/lib/injectable.dart
     - working/macros/example/lib/observable.dart
     - working/macros/example/lib/json_serializable.dart
     - working/macros/example/bin/user_main.dart