Use LibraryBuilder.target for imports/exports.

pkg/front_end/lib/src/fasta/dill/dill_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.dill_library_builder;
 
 import 'package:kernel/ast.dart'
     show Class, Field, Library, ListLiteral, Member, StaticGet, Typedef;
 
 import '../problems.dart' show unimplemented;
@@ skipping 22 matching lines @@
 
   final DillLoader loader;
 
   Library library;
 
   DillLibraryBuilder(this.uri, this.loader)
       : super(uri, new Scope.top(), new Scope.top());
 
   Uri get fileUri => uri;
 
+  @override
+  Library get target => library;
+
   void addClass(Class cls) {
     DillClassBuilder classBulder = new DillClassBuilder(cls, this);
     addBuilder(cls.name, classBulder, cls.fileOffset);
     cls.procedures.forEach(classBulder.addMember);
     cls.constructors.forEach(classBulder.addMember);
     for (Field field in cls.fields) {
       if (field.name.name == "_redirecting#") {
         ListLiteral initializer = field.initializer;
         for (StaticGet get in initializer.expressions) {
           RedirectingFactoryBody.restoreFromDill(get.target);
@@ skipping 56 matching lines @@
     // top-level declaration with the name `k` exists in `L`.
     if (builder.parent == this) return builder;
     return new KernelInvalidTypeBuilder(name, charOffset, fileUri);
   }
 
   @override
   String get fullNameForErrors {
     return library.name ?? "<library '${library.fileUri}'>";
   }
 }