Implement fullNameForErrors on all Builders.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index b676740f1dff32a5e5ca8d948782219946207fa6..12831a594f6b181a8de8f6c16a3c5a2b6e1c6eab 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -442,6 +442,9 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
 
   List<TypeVariableBuilder> copyTypeVariables(
       List<TypeVariableBuilder> original);
+
+  @override
+  String get fullNameForErrors => name ?? "<library '$relativeFileUri'>";
 }
 
 /// Unlike [Scope], this scope is used during construction of builders to