Add discovery service to stable APIs

generated/googleapis/test/youtube/v3_test.dart

 library googleapis.youtube.v3.test;
 
 import "dart:core" as core;
 import "dart:collection" as collection;
 import "dart:async" as async;
 import "dart:convert" as convert;
 
 import 'package:http/http.dart' as http;
 import 'package:http/testing.dart' as http_testing;
 import 'package:unittest/unittest.dart' as unittest;
@@ skipping 7472 matching lines @@
         return new async.Future.value(common_test.stringResponse(200, h, resp));
       }), true);
       res.unset(arg_channelId, onBehalfOfContentOwner: arg_onBehalfOfContentOwner).then(unittest.expectAsync((_) {}));
     });
 
   });
 
 
 }