Move the check for 'locals hides exports' to DillLibraryBuilder.buildAmbiguousBuilder().

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

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index ec866a9ee1e4a64e0076b2ba39ffea07ffb29729..e3067aa8ccbf980457e61dbfc39343eb82734d3b 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -98,11 +98,6 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
     Builder existing = map[name];
     if (existing == member) return false;
     if (existing != null) {
-      // For each entry mapping key `k` to declaration `d` in `NS` an entry
-      // mapping `k` to `d` is added to the exported namespace of `L` unless a
-      // top-level declaration with the name `k` exists in `L`.
-      if (existing.parent == this) return false;
-
       Builder result =
           buildAmbiguousBuilder(name, existing, member, -1, isExport: true);
       map[name] = result;