Implement fullNameForErrors on all Builders.

pkg/front_end/lib/src/fasta/builder/prefix_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.prefix_builder;
 
 import 'builder.dart' show Builder, LibraryBuilder, MemberBuilder;
 
 import '../messages.dart' show warning;
 
@@ skipping 40 matching lines @@
         Builder existing = exports[name];
         if (existing != null) {
           member = existing.combineAmbiguousImport(name, member, library);
         }
         exports[name] = member;
       });
       return this;
     }
     return super.combineAmbiguousImport(name, other, library);
   }
+
+  @override
+  String get fullNameForErrors => name;
 }