Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/native/ssa.dart

Index: pkg/compiler/lib/src/native/ssa.dart
diff --git a/pkg/compiler/lib/src/native/ssa.dart b/pkg/compiler/lib/src/native/ssa.dart
index 20d1ee311153192ae3cdc7f67cad5fe460f3f2e0..ed6656356576f043c7fb5297c8dd0a9402dc5dc0 100644
--- a/pkg/compiler/lib/src/native/ssa.dart
+++ b/pkg/compiler/lib/src/native/ssa.dart
@@ -33,7 +33,7 @@ void handleSsaNative(SsaBuilder builder, Expression nativeBody) {
         builder.graph.addConstant(arityConstant, builder.closedWorld);
     // TODO(ngeoffray): For static methods, we could pass a method with a
     // defined arity.
-    Element helper = backend.helpers.closureConverter;
+    MethodElement helper = backend.helpers.closureConverter;
     builder.pushInvokeStatic(nativeBody, helper, [local, arity]);
     HInstruction closure = builder.pop();
     return closure;