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

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

Index: pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart b/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
index 38626e198502a0bfc97c84bcdbcd4597524b2209..ffd32be6b4296a2b5d4edfc7ed6d4eab9fdcd274 100644
--- a/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
@@ -25,7 +25,8 @@ class PrefixBuilder extends Builder {
 
   final LibraryBuilder parent;
 
-  PrefixBuilder(this.name, this.exports, this.parent);
+  PrefixBuilder(this.name, this.exports, LibraryBuilder parent, int charOffset)
+      : parent = parent, super(parent, charOffset, parent.fileUri);
 
   Member findTopLevelMember(String name) {
     // TODO(ahe): Move this to KernelPrefixBuilder.