Modify backend strategy and how we access the closure converter.

pkg/compiler/lib/src/js_model/js_strategy.dart

Index: pkg/compiler/lib/src/js_model/js_strategy.dart
diff --git a/pkg/compiler/lib/src/js_model/js_strategy.dart b/pkg/compiler/lib/src/js_model/js_strategy.dart
index e3ea0863a755de386a432efbaf661f685e57d648..4173eaf4711c77704f5e4d64c0fb141b44c68c0f 100644
--- a/pkg/compiler/lib/src/js_model/js_strategy.dart
+++ b/pkg/compiler/lib/src/js_model/js_strategy.dart
@@ -5,6 +5,7 @@
 library dart2js.js_model.strategy;
 
 import '../backend_strategy.dart';
+import '../closure.dart' show ClosureConversionTask;
 import '../common_elements.dart';
 import '../compiler.dart';
 import '../elements/elements.dart' show TypedefElement;
@@ -109,7 +110,8 @@ class JsBackendStrategy implements BackendStrategy {
   }
 
   @override
-  void convertClosures(ClosedWorldRefiner closedWorldRefiner) {}
+  ClosureConversionTask createClosureConversionTask(Compiler compiler) =>
+      new KernelClosureConversionTask(compiler.measurer);
 
   @override
   SourceInformationStrategy get sourceInformationStrategy =>