Ensure locations are always provided, but don't store them yet.

pkg/front_end/lib/src/fasta/builder/member_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/member_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/member_builder.dart b/pkg/front_end/lib/src/fasta/builder/member_builder.dart
index f4cb6f1bfeea797284ae45a77f49b587c2fbce50..fc63ed9023cea1c27ec51d7363363f04394e0dab 100644
--- a/pkg/front_end/lib/src/fasta/builder/member_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/member_builder.dart
@@ -10,8 +10,14 @@ import 'builder.dart' show
     ModifierBuilder;
 
 abstract class MemberBuilder extends ModifierBuilder {
+  /// For top-level members, the parent is set correctly during
+  /// construction. However, for class members, the parent is initially the
+  /// library and updated later.
   Builder parent;
 
+  MemberBuilder(Builder parent, int charOffset)
+      : parent = parent, super(parent, charOffset);
+
   bool get isInstanceMember => isClassMember && !isStatic;
 
   bool get isClassMember => parent is ClassBuilder;