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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.library_builder;
 
 import '../combinator.dart' show Combinator;
 
 import '../errors.dart' show InputError, internalError, printUnexpected;
 
@@ skipping 80 matching lines @@
 
   /// Returns true if the export scope was modified.
   bool addToExportScope(String name, Builder member) {
     if (name.startsWith("_")) return false;
     if (member is PrefixBuilder) return false;
     Map<String, Builder> map =
         member.isSetter ? exports.setters : exports.local;
     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;
       return result != existing;
     } else {
       map[name] = member;
     }
     return true;
   }
 
@@ skipping 51 matching lines @@
   /// Don't use for scope lookup. Only use when an element is known to exist
   /// (and not a setter).
   Builder operator [](String name) {
     return scope.local[name] ?? internalError("Not found: '$name'.");
   }
 
   Builder lookup(String name, int charOffset, Uri fileUri) {
     return scope.lookup(name, charOffset, fileUri);
   }
 }