Error on part of self.

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 6c992f6654fd75c32bbefd244316ff96d96e3095..5bc51159d49a3500ed579b5119c889c5adc8b744 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
@@ -334,7 +334,11 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
 
   void includeParts() {
     for (SourceLibraryBuilder<T, R> part in parts.toList()) {
-      includePart(part);
+      if (part == this) {
+        addCompileTimeError(-1, "A file can't be a part of itself.");
+      } else {
+        includePart(part);
+      }
     }
   }