Implement fullNameForErrors on all Builders.

pkg/front_end/lib/src/fasta/builder/mixed_accessor.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.mixed_accessor;
 
 import 'builder.dart' show Builder, LibraryBuilder;
 
 /// Represents the import of a getter and setter from two different libraries.
 class MixedAccessor extends Builder {
   final Builder getter;
   final Builder setter;
 
   MixedAccessor(this.getter, this.setter, LibraryBuilder parent)
       : super(
             parent,
             -1, // Synthetic element has no charOffset.
             parent.fileUri) {
     next = getter;
   }
+
+  @override
+  String get fullNameForErrors => getter.fullNameForErrors;
 }