The mirror tests made the invalid assumption that the library with the tests

tests/lib/mirrors/mirrors_test.dart

 // Copyright (c) 2011, 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.
 
 // TODO(rmacnak): Move the existing mirror tests here (a place for 
 // cross-implementation tests).
 
 #library("MirrorsTest.dart");
 #import("dart:mirrors");
 #import("../../../lib/unittest/unittest.dart");
 
 var topLevelField;
 
 class Class {
   var field;
   static var staticField;
 }
 
 testFieldAccess(mirrors) {
   var instance = new Class();
 
-  var libMirror = mirrors.rootLibrary;
+  var libMirror = mirrors.libraries()["MirrorsTest.dart"];
   var classMirror = libMirror.classes()["Class"];
   var instMirror = mirrors.mirrorOf(instance);
 
   libMirror.setField('topLevelField', 42);
   var future = libMirror.getField('topLevelField');
   future.then(expectAsync1((resultMirror) {
     expect(resultMirror.reflectee, equals(42));
     expect(topLevelField, equals(42));   
   }));
   
@@ skipping 11 matching lines @@
     expect(instance.field, equals(44)); 
   }));
 }
 
 main() {
   var mirrors = currentMirrorSystem();
 
   test("Test field access", () { testFieldAccess(mirrors); });
 }