Fix warning after loader refactor discovered by a red bot.

pkg/compiler/lib/src/library_loader.dart

 // Copyright (c) 2012, 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 dart2js.library_loader;
 
 import 'dart:async';
 
 import 'common.dart';
 import 'common/names.dart' show
@@ skipping 559 matching lines @@
    * Handle a part tag in the scope of [library]. The [resolvedUri] given is
    * used as is, any URI resolution should be done beforehand.
    */
   Future scanPart(Part part, Uri resolvedUri, LibraryElement library) {
     if (!resolvedUri.isAbsolute) throw new ArgumentError(resolvedUri);
     Uri readableUri = uriTranslator.translate(library, resolvedUri, part);
     if (readableUri == null) return new Future.value();
     return reporter.withCurrentElement(library, () {
       return scriptLoader.readScript(readableUri, part).then((Script script) {
         if (script == null) return;
-        return createUnitSync(script, library);
+        createUnitSync(script, library);
       });
     });
   }
 
   /**
    * Handle an import/export tag by loading the referenced library and
    * registering its dependency in [handler] for the computation of the import/
    * export scope. If the tag does not contain a valid URI, then its dependency
    * is not registered in [handler].
    */
@@ skipping 926 matching lines @@
   /// Called after a request to load a library. The [results] will include all
   /// transitive libraries loaded as a result of the initial request.
   Future onLibrariesLoaded(LoadedLibraries results);
 
   /// Called whenever a library element is created.
   void onLibraryCreated(LibraryElement library);
 
   /// Called whenever a library is scanned from a script file.
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader);
 }