Use LibraryBuilder.target for imports/exports.

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 '../problems.dart' show internalProblem;
 
@@ skipping 47 matching lines @@
 
   LibraryBuilder(Uri fileUri, this.scope, this.exportScope)
       : fileUri = fileUri,
         relativeFileUri = relativizeUri(fileUri),
         scopeBuilder = new ScopeBuilder(scope),
         exportScopeBuilder = new ScopeBuilder(exportScope),
         super(null, -1, fileUri);
 
   Loader get loader;
 
+  @override
+  R get target;
+
   Uri get uri;
 
   Builder addBuilder(String name, Builder builder, int charOffset);
 
   void addExporter(
       LibraryBuilder exporter, List<Combinator> combinators, int charOffset) {
     exporters.add(new Export(exporter, this, combinators, charOffset));
   }
 
   /// See `Loader.addCompileTimeError` for an explanation of the
@@ skipping 115 matching lines @@
             templateInternalProblemNotFoundIn.withArguments(
                 name, relativeFileUri),
             -1,
             null);
   }
 
   Builder lookup(String name, int charOffset, Uri fileUri) {
     return scope.lookup(name, charOffset, fileUri);
   }
 }