Implement fullNameForErrors on all Builders.

pkg/front_end/lib/src/fasta/dill/dill_library_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.dill_library_builder;
 
 import 'package:kernel/ast.dart'
     show
         Class,
         ExpressionStatement,
@@ skipping 91 matching lines @@
   }
 
   void addToScope(String name, Builder member) {
     internalError("Not implemented yet.");
   }
 
   KernelInvalidTypeBuilder buildAmbiguousBuilder(
       String name, Builder builder, Builder other, int charOffset) {
     return new KernelInvalidTypeBuilder(name, charOffset, fileUri);
   }
+
+  @override
+  String get fullNameForErrors {
+    return library.name ?? "<library '${library.fileUri}'>";
+  }
 }