Fix issue 21474 in mirrors.

runtime/lib/mirrors_impl.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.
 
 // VM-specific implementation of the dart:mirrors library.
 
 import "dart:collection" show UnmodifiableListView, UnmodifiableMapView;
 
 final emptyList = new UnmodifiableListView([]);
 final emptyMap = new UnmodifiableMapView({});
@@ skipping 755 matching lines @@
     decls.addAll(_constructors);
     typeVariables.forEach((tv) => decls[tv.simpleName] = tv);
     return _declarations =
         new UnmodifiableMapView<Symbol, DeclarationMirror>(decls);
   }
 
   Map<Symbol, Mirror> _cachedMembers;
   Map<Symbol, Mirror> get _members {
     if (_cachedMembers == null) {
       var whoseMembers = _isMixinAlias ? _trueSuperclass : this;
-      _cachedMembers = _makeMemberMap(mixin._computeMembers(whoseMembers._reflectee));
+      _cachedMembers = _makeMemberMap(mixin._computeMembers(whoseMembers.mixin._reflectee));
     }
     return _cachedMembers;
   }
 
   Map<Symbol, MethodMirror> _cachedMethods;
   Map<Symbol, MethodMirror> get _methods {
     if (_cachedMethods == null) {
       _cachedMethods = _filterMap(
           _members,
           (key, value) => (value is MethodMirror && value.isRegularMethod));
@@ skipping 913 matching lines @@
     if (typeMirror == null) {
       typeMirror = makeLocalTypeMirror(key);
       _instanitationCache[key] = typeMirror;
       if (typeMirror is ClassMirror && !typeMirror._isGeneric) {
         _declarationCache[key] = typeMirror;
       }
     }
     return typeMirror;
   }
 }