Compute correct parent and fileUri for top-level fields.

pkg/front_end/lib/src/fasta/kernel/kernel_variable_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_variable_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_variable_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_variable_builder.dart
index afc7bda670c4161fb2fe023b6bbb7917ee26ca10..48c86c98b9ce1c32741224b0ee529733c82b0be7 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_variable_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_variable_builder.dart
@@ -13,9 +13,10 @@ import 'kernel_builder.dart' show
 class KernelVariableBuilder extends Builder {
   final VariableDeclaration variable;
 
-  KernelVariableBuilder(VariableDeclaration variable, Builder parent)
+  KernelVariableBuilder(
+      VariableDeclaration variable, Builder parent, Uri fileUri)
       : variable = variable,
-        super(parent, variable.fileOffset, parent.fileUri);
+        super(parent, variable.fileOffset, fileUri);
 
   bool get isLocal => true;