Add NativeEnqueuer to work with the Enqueuer.

sdk/lib/_internal/compiler/implementation/compiler.dart

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compiler.dart b/sdk/lib/_internal/compiler/implementation/compiler.dart
index 774c2c6ea2ee79119c8b246b669bb2a8f42aed93..7b5569f4d7749e172ec351ad91a77752e84f35e1 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -74,8 +74,12 @@ abstract class Backend {
 
   void enqueueHelpers(Enqueuer world);
   void codegen(WorkItem work);
-  void processNativeClasses(Enqueuer world,
-                            Collection<LibraryElement> libraries);
+
+  // The backend determines the native resolution enqueuer so tools like
+  // dart2dart can ignore the native classes.
+  NativeEnqueuer nativeResolutionEnqueuer(world);
+  NativeEnqueuer nativeCodegenEnqueuer(world);
+
   void assembleProgram();
   List<CompilerTask> get tasks;
 
@@ -521,11 +525,18 @@ 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);
     processQueue(enqueuer.resolution, main);
     log('Resolved ${enqueuer.resolution.resolvedElements.length} elements.');
+    log('Resolved ${enqueuer.resolution.nativeEnqueuer.registeredClasses.length} native elements used, '
+        '${enqueuer.resolution.nativeEnqueuer.unusedClasses.length} native elements dead.');
 
     if (compilationFailed) return;
 
@@ -540,6 +551,8 @@ abstract class Compiler implements DiagnosticListener {
     phase = PHASE_COMPILING;
     processQueue(enqueuer.codegen, main);
     log('Compiled ${codegenWorld.generatedCode.length} methods.');
+    log('Compiled ${enqueuer.codegen.nativeEnqueuer.registeredClasses.length} native classes, '
+        '${enqueuer.codegen.nativeEnqueuer.unusedClasses.length} native classes omitted.');
 
     if (compilationFailed) return;
 
@@ -549,7 +562,7 @@ 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) {
@@ -652,6 +665,7 @@ abstract class Compiler implements DiagnosticListener {
     TreeElements result = world.getCachedElements(element);
     if (result != null) return result;
     if (!identical(world, enqueuer.resolution)) {
+      throw 'Internal error: unresolved element: $element.';
       internalErrorOnElement(element,
                              'Internal error: unresolved element: $element.');
     }