Implement fullNameForErrors on all Builders.

pkg/front_end/lib/src/fasta/kernel/kernel_variable_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_variable_builder;
 
 import 'package:kernel/ast.dart' show VariableDeclaration;
 
 import 'kernel_builder.dart' show Builder;
 
 class KernelVariableBuilder extends Builder {
   final VariableDeclaration variable;
 
   KernelVariableBuilder(
       VariableDeclaration variable, Builder parent, Uri fileUri)
       : variable = variable,
         super(parent, variable.fileOffset, fileUri);
 
   bool get isLocal => true;
 
   VariableDeclaration get target => variable;
+
+  @override
+  String get fullNameForErrors => variable.name ?? "<unnamed>";
 }