Use entities in ClosedWorld interface

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

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index c3f2a729dd791a5d677e951635813d221e18294b..5678b81f77a5189ea24d7a3c1f37bb227c41fc6f 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -1632,14 +1632,15 @@ class SimpleTypeInferrerVisitor<T>
       return length.value;
     } else if (element != null &&
         element.isField &&
-        Elements.isStaticOrTopLevelField(element) &&
-        closedWorld.fieldNeverChanges(element)) {
+        Elements.isStaticOrTopLevelField(element)) {
       FieldElement fieldElement = element;
-      ConstantValue value =
-          compiler.backend.constants.getConstantValue(fieldElement.constant);
-      if (value != null && value.isInt) {
-        IntConstantValue intValue = value;
-        return intValue.primitiveValue;
+      if (closedWorld.fieldNeverChanges(fieldElement)) {
+        ConstantValue value =
+            compiler.backend.constants.getConstantValue(fieldElement.constant);
+        if (value != null && value.isInt) {
+          IntConstantValue intValue = value;
+          return intValue.primitiveValue;
+        }
       }
     }
     return null;