anchor.diff:        } else {
           unhandled("${node.runtimeType}", "finalizeExports", -1, fileUri);
         }
  -      var library = loader.read(libraryUri, -1);
  +      DillLibraryBuilder library = loader.builders[libraryUri];
  +      if (library == null) {
  +        internalProblem(
  +            templateUnspecified.withArguments("No builder for '$libraryUri'."),
  +            -1,
  +            fileUri);
  +      }
         Builder builder;
         if (isSetter) {
           builder = library.exportScope.setters[name];
  
positive.code:         builder = library.exportScope.local[name];
          exportScopeBuilder.addMember(name, builder);
        }
        assert(node == builder.target);
      }
    }
  
negative.code: 
  import '../fasta_codes.dart' show templateUnspecified;
  
  import '../problems.dart' show internalProblem, unhandled, unimplemented;
  
  import '../kernel/kernel_builder.dart'
      show
  