Make Enqueuer a pure interface.

tests/compiler/dart2js/mirror_tree_shaking_test.dart

Index: tests/compiler/dart2js/mirror_tree_shaking_test.dart
diff --git a/tests/compiler/dart2js/mirror_tree_shaking_test.dart b/tests/compiler/dart2js/mirror_tree_shaking_test.dart
index 2b6950144db8116262620d613ce75465b21f59cb..34f2b69943b7d3914a13047e13fd7aa7c94b205b 100644
--- a/tests/compiler/dart2js/mirror_tree_shaking_test.dart
+++ b/tests/compiler/dart2js/mirror_tree_shaking_test.dart
@@ -23,10 +23,10 @@ main() {
     Expect.isFalse(compiler.enqueuer.resolution.hasEnqueuedReflectiveElements);
     Expect.isFalse(
         compiler.enqueuer.resolution.hasEnqueuedReflectiveStaticFields);
-    Expect.isFalse(compiler.enqueuer.codegen.hasEnqueuedReflectiveElements);
-    Expect.isFalse(compiler.enqueuer.codegen.hasEnqueuedReflectiveStaticFields);
-    Expect.isFalse(compiler.disableTypeInference);
     JavaScriptBackend backend = compiler.backend;
+    Expect.isFalse(backend.codegenEnqueuer.hasEnqueuedReflectiveElements);
+    Expect.isFalse(backend.codegenEnqueuer.hasEnqueuedReflectiveStaticFields);
+    Expect.isFalse(compiler.disableTypeInference);
     Expect.isFalse(backend.hasRetainedMetadata);
   });
 }