Reapply (plus add some more) Entity-ify some portions of LocalsHandler and Closure.

tests/compiler/dart2js/serialization/model_test_helper.dart

Index: tests/compiler/dart2js/serialization/model_test_helper.dart
diff --git a/tests/compiler/dart2js/serialization/model_test_helper.dart b/tests/compiler/dart2js/serialization/model_test_helper.dart
index 33ec0df56b4028c0257387ed9460c98711199230..b23cb1ecba308ed4b176555d196d269a1bd2e475 100644
--- a/tests/compiler/dart2js/serialization/model_test_helper.dart
+++ b/tests/compiler/dart2js/serialization/model_test_helper.dart
@@ -241,7 +241,7 @@ bool areLocalsEquivalent(Local a, Local b) {
   }
 }
 
-bool areCapturedVariablesEquivalent(CapturedVariable a, CapturedVariable b) {
+bool areCapturedVariablesEquivalent(FieldEntity a, FieldEntity b) {
   if (a == b) return true;
   if (a == null || b == null) return false;
   if (a is ClosureFieldElement && b is ClosureFieldElement) {