Implement fullNameForErrors on all Builders.

pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
index 56eee91ae0100fb0db8f5a3f44984d341fe25129..30a175648d611007afeaf56d179ffad4fd6783cc 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
@@ -109,4 +109,9 @@ class DillLibraryBuilder extends LibraryBuilder<KernelTypeBuilder, Library> {
       String name, Builder builder, Builder other, int charOffset) {
     return new KernelInvalidTypeBuilder(name, charOffset, fileUri);
   }
+
+  @override
+  String get fullNameForErrors {
+    return library.name ?? "<library '${library.fileUri}'>";
+  }
 }