Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart

 // Copyright (c) 2016, 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.
 
 library fasta.kernel_class_builder;
 
 import 'package:kernel/ast.dart'
     show
         Class,
         Constructor,
@@ skipping 254 matching lines @@
               declaredMember.fileOffset,
               "The method '$name::${declaredMember.name.name}' doesn't have "
               "the named parameter '${interfaceNamedParameters.current.name}' "
               "of overriden method '${interfaceMember.enclosingClass.name}::"
               "${interfaceMember.name.name}'.");
           break outer;
         }
       }
     }
   }
+
+  String get fullNameForErrors {
+    return isMixinApplication
+        ? "${supertype.fullNameForErrors} with ${mixedInType.fullNameForErrors}"
+        : name;
+  }
 }