Use LibraryBuilder.target for imports/exports.

pkg/front_end/lib/src/fasta/kernel/kernel_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.kernel_library_builder;
 
-import 'package:front_end/src/fasta/dill/dill_library_builder.dart';
 import 'package:front_end/src/fasta/combinator.dart' as fasta;
 import 'package:front_end/src/fasta/export.dart';
 import 'package:front_end/src/fasta/import.dart';
 import 'package:kernel/ast.dart';
 
 import 'package:kernel/clone.dart' show CloneVisitor;
 
 import '../../scanner/token.dart' show Token;
 
 import '../fasta_codes.dart'
@@ skipping 71 matching lines @@
 
   final List<KernelProcedureBuilder> nativeMethods = <KernelProcedureBuilder>[];
 
   final List<KernelTypeVariableBuilder> boundlessTypeVariables =
       <KernelTypeVariableBuilder>[];
 
   KernelLibraryBuilder(Uri uri, Uri fileUri, Loader loader, this.isPatch)
       : library = new Library(uri, fileUri: relativizeUri(fileUri)),
         super(loader, fileUri);
 
+  @override
   Library get target => library;
 
   Uri get uri => library.importUri;
 
   KernelTypeBuilder addNamedType(
       String name, List<KernelTypeBuilder> arguments, int charOffset) {
     return addType(
         new KernelNamedTypeBuilder(name, arguments, charOffset, fileUri));
   }
 
@@ skipping 639 matching lines @@
         Iterable<fasta.Combinator> fastaCombinators) {
       return fastaCombinators?.map((c) {
         List<String> nameList = c.names.toList();
         return c.isShow
             ? new Combinator.show(nameList)
             : new Combinator.hide(nameList);
       })?.toList();
     }
 
     for (Import import in imports) {
-      var importedBuilder = import.imported;
-      Library importedLibrary;
-      if (importedBuilder is DillLibraryBuilder) {
-        importedLibrary = importedBuilder.library;
-      } else if (importedBuilder is KernelLibraryBuilder) {
-        importedLibrary = importedBuilder.library;
-      }
+      Library importedLibrary = import.imported.target;
       if (importedLibrary != null) {
         library.addDependency(new LibraryDependency.import(importedLibrary,
             name: import.prefix,
             combinators: toKernelCombinators(import.combinators)));
       }
     }
 
     for (Export export in exports) {
-      var exportedBuilder = export.exported;
-      Library exportedLibrary;
-      if (exportedBuilder is DillLibraryBuilder) {
-        exportedLibrary = exportedBuilder.library;
-      } else if (exportedBuilder is KernelLibraryBuilder) {
-        exportedLibrary = exportedBuilder.library;
-      }
+      Library exportedLibrary = export.exported.target;
       if (exportedLibrary != null) {
         library.addDependency(new LibraryDependency.export(exportedLibrary,
             combinators: toKernelCombinators(export.combinators)));
       }
     }
 
     for (var part in parts) {
       // TODO(scheglov): Add support for annotations, see
       // https://github.com/dart-lang/sdk/issues/30284.
       String fileUri = part.fileUri.toString();
@@ skipping 166 matching lines @@
           mixinApplicationClasses.putIfAbsent(name, () => builder);
       if (existing != builder) {
         part.scope.local.remove(name);
       }
     });
     super.includePart(part);
     nativeMethods.addAll(part.nativeMethods);
     boundlessTypeVariables.addAll(part.boundlessTypeVariables);
   }
 }