Add NativeEnqueuer to work with the Enqueuer.

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index 97fad390ba7e5d24a8db82a708960b1d2493c0e1..d23ada5eeebc2fec40192a193a3c5317d5a1b95b 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -74,8 +74,12 @@ abstract class Backend {
 
   abstract void enqueueHelpers(Enqueuer world);
   abstract void codegen(WorkItem work);
-  abstract void processNativeClasses(Enqueuer world,
-                                     Collection<LibraryElement> libraries);
+
+  // The backend determines the native resolution enqueuer so tools like
+  // dart2dart can ignore the native classes.
+  abstract NativeEnqueuer nativeResolutionEnqueuer(world);
+  abstract NativeEnqueuer nativeCodegenEnqueuer(world);
+
   abstract void assembleProgram();
   abstract List<CompilerTask> get tasks;
 
@@ -484,6 +488,11 @@ abstract class Compiler implements DiagnosticListener {
       });
     }
 
+    enqueuer.resolution.nativeEnqueuer =
+        backend.nativeResolutionEnqueuer(enqueuer.resolution);
+    enqueuer.codegen.nativeEnqueuer =
+        backend.nativeCodegenEnqueuer(enqueuer.codegen);
+
     log('Resolving...');
     phase = PHASE_RESOLVING;
     backend.enqueueHelpers(enqueuer.resolution);
@@ -512,7 +521,7 @@ abstract class Compiler implements DiagnosticListener {
   }
 
   void processQueue(Enqueuer world, Element main) {
-    backend.processNativeClasses(world, libraries.values);
+    world.nativeEnqueuer.processNativeClasses(libraries.values);
     world.addToWorkList(main);
     progress.reset();
     world.forEach((WorkItem work) {