feat(router): add interfaces for route definitions in RouteConfig

Closes #2261

modules/angular2/src/router/route_definition.dart

@@ -0,0 +1,7 @@
+library angular2.src.router.route_definition;
+
+abstract class RouteDefinition {
+  final String path;
+  final String as;
+  const RouteDefinition({this.path, this.as});
+}