Use backend-specific enqueuer.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index f11ed37801339a13b2d052318cb1c521c2c7e9a7..c4c357c72854cee67dee6e24b46e8dad5e1af788 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -27,7 +27,8 @@ import '../dart_types.dart';
 import '../deferred_load.dart' show DeferredLoadTask;
 import '../dump_info.dart' show DumpInfoTask;
 import '../elements/elements.dart';
-import '../enqueue.dart' show Enqueuer, ResolutionEnqueuer;
+import '../enqueue.dart'
+    show Enqueuer, ResolutionEnqueuer, TreeShakingEnqueuerStrategy;
 import '../io/position_information.dart' show PositionSourceInformationStrategy;
 import '../io/source_information.dart' show SourceInformationStrategy;
 import '../io/start_end_information.dart'
@@ -64,6 +65,7 @@ import 'backend_serialization.dart' show JavaScriptBackendSerialization;
 import 'checked_mode_helpers.dart';
 import 'constant_handler_javascript.dart';
 import 'custom_elements_analysis.dart';
+import 'enqueuer.dart';
 import 'js_interop_analysis.dart' show JsInteropAnalysis;
 import 'lookup_map_analysis.dart' show LookupMapAnalysis;
 import 'namer.dart';
@@ -400,7 +402,6 @@ class JavaScriptBackend extends Backend {
     return _fixedArrayTypeCache;
   }
 
-
   /// Maps special classes to their implementation (JSXxx) class.
   Map<ClassElement, ClassElement> implementationClasses;
 
@@ -1563,6 +1564,11 @@ class JavaScriptBackend extends Backend {
     }
   }
 
+  CodegenEnqueuer createCodegenEnqueuer(Compiler compiler) {
+    return new CodegenEnqueuer(compiler, createItemCompilationContext,
+        const TreeShakingEnqueuerStrategy());
+  }
+
   WorldImpact codegen(CodegenWorkItem work) {
     Element element = work.element;
     if (compiler.elementHasCompileTimeError(element)) {