Split World usage into open, inference, and closed world.

pkg/compiler/lib/src/js_backend/namer.dart

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 0d43e7b05b6291cf873de576faf28f83dd31a5bd..7118b7f0a1f74308296e10f97403bce4d5df73ae 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -853,7 +853,7 @@ class Namer {
     // mangle the field names of classes extending native classes.
     // Methods on such classes are stored on the interceptor, not the instance,
     // so only fields have the potential to clash with a native property name.
-    ClassWorld classWorld = compiler.world;
+    ClassWorld classWorld = compiler.closedWorld;
     if (classWorld.isUsedAsMixin(enclosingClass) ||
         _isShadowingSuperField(element) ||
         _isUserClassExtendingNative(enclosingClass)) {