Aparently co19 tests have their own separate annotation parsing.

tools/testing/dart/test_suite.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 1834 matching lines @@
 
     return {
       "vmOptions": <List>[[]],
       "sharedOptions": <String>[],
       "dartOptions": null,
       "packageRoot": null,
       "hasCompileError": hasCompileError,
       "hasRuntimeError": hasRuntimeError,
       "hasStaticWarning": hasStaticWarning,
       "otherScripts": <String>[],
+      "otherResources": <String>[],
       "isMultitest": isMultitest,
       "isMultiHtmlTest": false,
       "subtestNames": <String>[],
       "isolateStubs": '',
       "containsDomImport": false,
     };
   }
 }
 
 /// Used for testing packages in on off settings, i.e., we pass in the actual
@@ skipping 638 matching lines @@
       }
       if (path.length > WINDOWS_SHORTEN_PATH_LIMIT) {
         ++shortNameCounter;
         var pathEnd = path.substring(path.length - WINDOWS_PATH_END_LENGTH);
         path = "short${shortNameCounter}_$pathEnd";
       }
     }
     return path;
   }
 }