Add NativeEnqueuer to work with the Enqueuer.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.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.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 196 matching lines @@
     final coreLibrary = compiler.coreLibrary;
     for (final name in LITERAL_TYPE_NAMES) {
       ClassElement classElement = coreLibrary.findLocal(new SourceString(name));
       classElement.ensureResolved(compiler);
     }
   }
   void codegen(WorkItem work) { }
   void processNativeClasses(Enqueuer world,
                             Collection<LibraryElement> libraries) { }
 
+  // Stub native enqueuers.
+  NativeEnqueuer nativeResolutionEnqueuer(Enqueuer world) => new NativeEnqueuer();
+  NativeEnqueuer nativeCodegenEnqueuer(Enqueuer world) => new NativeEnqueuer();
+
   bool isUserLibrary(LibraryElement lib) {
     final INTERNAL_HELPERS = [
       compiler.jsHelperLibrary,
       compiler.interceptorsLibrary,
     ];
     return INTERNAL_HELPERS.indexOf(lib) == -1 && !lib.isPlatformLibrary;
   }
 
   void assembleProgram() {
     // Conservatively traverse all platform libraries and collect member names.
@@ skipping 328 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);