Add discovery service to stable APIs

generated/googleapis/test/mirror/v1_test.dart

 library googleapis.mirror.v1.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 2174 matching lines @@
       res.list(arg_itemId).then(unittest.expectAsync(((api.AttachmentsListResponse response) {
         checkAttachmentsListResponse(response);
       })));
     });
 
   });
 
 
 }