Access NativeData only through ClosedWorld

pkg/compiler/lib/src/js_emitter/native_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/native_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/native_emitter.dart b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
index 4dbceb87fdffeed23f9f835d72838b5ea68065ac..a5551c2decf0301fc8d11b8161593a91314c6ba8 100644
--- a/pkg/compiler/lib/src/js_emitter/native_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
@@ -20,6 +20,7 @@ import 'model.dart';
 
 class NativeEmitter {
   final CodeEmitterTask emitterTask;
+  final NativeData nativeData;
 
   // Whether the application contains native classes.
   bool hasNativeClasses = false;
@@ -35,7 +36,7 @@ class NativeEmitter {
   // Caches the methods that have a native body.
   Set<FunctionEntity> nativeMethods = new Set<FunctionEntity>();
 
-  NativeEmitter(CodeEmitterTask emitterTask) : this.emitterTask = emitterTask;
+  NativeEmitter(this.emitterTask, this.nativeData);
 
   Compiler get compiler => emitterTask.compiler;
 
@@ -43,8 +44,6 @@ class NativeEmitter {
 
   CodegenWorldBuilder get worldBuilder => compiler.codegenWorldBuilder;
 
-  NativeData get nativeData => backend.nativeData;
-
   InterceptorData get interceptorData => backend.interceptorData;
 
   Namer get namer => backend.namer;