Correctly resolve package parts.

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 934b98092526ce2a7b811d3fbff0ea7dd465f94b..1af020ca3a217fdd8fac118726829e447fd735da 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
@@ -131,7 +131,8 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
       resolvedUri = new Uri(scheme: "dart", path: "${uri.path}/$path");
       newFileUri = fileUri.resolve(path);
     } else {
-      newFileUri = resolvedUri = resolve(path);
+      resolvedUri = uri.resolve(path);
+      newFileUri = fileUri.resolve(path);
     }
     parts.add(loader.read(resolvedUri, newFileUri));
   }