Handle top-level getters and setters in 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 97331f83ccb5675bd049af062f01ec1f11cf05d7..4da1f859e4b80b894a481c9af7bc7867f2a5433e 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
@@ -283,7 +283,11 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
       }
     }
     part.members.forEach((String name, Builder builder) {
-      addBuilder(name, builder, -1);
+      if (builder.next != null) {
+        assert(builder.next.next == null);
+        addBuilder(name, builder.next, builder.next.charOffset);
+      }
+      addBuilder(name, builder, builder.charOffset);
     });
     types.addAll(part.types);
     constructorReferences.addAll(part.constructorReferences);