Access BackendUsage through ClosedWorld

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

Index: pkg/compiler/lib/src/js_emitter/lazy_emitter/model_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/lazy_emitter/model_emitter.dart b/pkg/compiler/lib/src/js_emitter/lazy_emitter/model_emitter.dart
index 3c644a638f09da7e420e395dec6f7a316735eba4..6f4f899e4b68fd78172d5f7dacf9b515c2e810fe 100644
--- a/pkg/compiler/lib/src/js_emitter/lazy_emitter/model_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/lazy_emitter/model_emitter.dart
@@ -253,10 +253,11 @@ class ModelEmitter {
     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; })", []));