Access BackendUsage through ClosedWorld

pkg/compiler/lib/src/js_emitter/lazy_emitter/model_emitter.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.js_emitter.lazy_emitter.model_emitter;
 
 import 'package:js_runtime/shared/embedded_names.dart'
     show
         CREATE_NEW_ISOLATE,
         DEFERRED_LIBRARY_URIS,
@@ skipping 235 matching lines @@
 
     holes.addAll(nativeHoles(program));
 
     return js.js.statement(boilerplate, holes);
   }
 
   Map<String, dynamic> nativeHoles(Program program) {
     Map<String, dynamic> nativeHoles = <String, dynamic>{};
 
     js.Statement nativeIsolateAffinityTagInitialization;
-    if (NativeGenerator.needsIsolateAffinityTagInitialization(backend)) {
+    if (NativeGenerator
+        .needsIsolateAffinityTagInitialization(_closedWorld.backendUsage)) {
       nativeIsolateAffinityTagInitialization =
           NativeGenerator.generateIsolateAffinityTagInitialization(
-              backend,
+              _closedWorld.backendUsage,
               generateEmbeddedGlobalAccess,
               // TODO(floitsch): internStringFunction.
               js.js("(function(x) { return x; })", []));
     } else {
       nativeIsolateAffinityTagInitialization = js.js.statement(";");
     }
     nativeHoles['nativeIsolateAffinityTagInitialization'] =
         nativeIsolateAffinityTagInitialization;
 
     js.Expression nativeInfoAccess = js.js('nativeInfo', []);
@@ skipping 1016 matching lines @@
   #eagerClasses;
 
   var end = Date.now();
   // print('Setup: ' + (end - start) + ' ms.');
 
   #invokeMain;  // Start main.
 
 })(Date.now(), #code)
 }""";
 }