Compute NativeBehavior for native methods and fields.

pkg/compiler/lib/src/inferrer/closure_tracer.dart

Index: pkg/compiler/lib/src/inferrer/closure_tracer.dart
diff --git a/pkg/compiler/lib/src/inferrer/closure_tracer.dart b/pkg/compiler/lib/src/inferrer/closure_tracer.dart
index bb406ca6c231d023bf2c246d0841f18541dd0025..a757a7b89f4e98ac28e9cfb7ce47b027b895e6a5 100644
--- a/pkg/compiler/lib/src/inferrer/closure_tracer.dart
+++ b/pkg/compiler/lib/src/inferrer/closure_tracer.dart
@@ -6,6 +6,7 @@ library compiler.src.inferrer.closure_tracer;
 
 import '../common/names.dart' show Names;
 import '../elements/elements.dart';
+import '../js_backend/backend_helpers.dart';
 import '../types/types.dart' show TypeMask;
 import '../universe/selector.dart' show Selector;
 import 'debug.dart' as debug;
@@ -71,7 +72,7 @@ class ClosureTracerVisitor extends TracerVisitor<ApplyableTypeInformation> {
     Element called = info.calledElement;
     if (compiler.backend.isForeign(called)) {
       String name = called.name;
-      if (name == 'JS' || name == 'DART_CLOSURE_TO_JS') {
+      if (name == BackendHelpers.JS || name == 'DART_CLOSURE_TO_JS') {
         bailout('Used in JS ${info.call}');
       }
     }