Compute NativeBehavior for native methods and fields.

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

Index: pkg/compiler/lib/src/inferrer/map_tracer.dart
diff --git a/pkg/compiler/lib/src/inferrer/map_tracer.dart b/pkg/compiler/lib/src/inferrer/map_tracer.dart
index 22a9f0f4c7e9910ee2416096f01de13e19cc5923..0abb2ca51cedb99a878da9c0a4a5f1cd06aa881f 100644
--- a/pkg/compiler/lib/src/inferrer/map_tracer.dart
+++ b/pkg/compiler/lib/src/inferrer/map_tracer.dart
@@ -5,6 +5,7 @@
 library compiler.src.inferrer.map_tracer;
 
 import '../elements/elements.dart';
+import '../js_backend/backend_helpers.dart';
 import '../universe/selector.dart' show Selector;
 import 'node_tracer.dart';
 import 'type_graph_nodes.dart';
@@ -66,7 +67,8 @@ class MapTracerVisitor extends TracerVisitor<MapTypeInformation> {
   visitStaticCallSiteTypeInformation(StaticCallSiteTypeInformation info) {
     super.visitStaticCallSiteTypeInformation(info);
     Element called = info.calledElement;
-    if (compiler.backend.isForeign(called) && called.name == 'JS') {
+    if (compiler.backend.isForeign(called) &&
+        called.name == BackendHelpers.JS) {
       bailout('Used in JS ${info.call}');
     }
   }