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

tests/compiler/dart2js/serialization/model_test_helper.dart

 // Copyright (c) 2015, 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.serialization_model_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/closure.dart';
@@ skipping 223 matching lines @@
   if (a == null || b == null) return false;
 
   if (a is Element) {
     return b is Element && areElementsEquivalent(a as Element, b as Element);
   } else {
     return a.runtimeType == b.runtimeType &&
         areElementsEquivalent(a.executableContext, b.executableContext);
   }
 }
 
-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) {
     return areElementsEquivalent(a.closureClass, b.closureClass) &&
         areLocalsEquivalent(a.local, b.local);
   } else if (a is BoxFieldElement && b is BoxFieldElement) {
     return areElementsEquivalent(a.variableElement, b.variableElement) &&
         areLocalsEquivalent(a.box, b.box);
   }
   return false;
@@ skipping 39 matching lines @@
   check(outputUnit1, outputUnit2, 'OutputUnit.isMainOutput $message',
       outputUnit1.isMainOutput, outputUnit2.isMainOutput);
   checkSetEquivalence(
       outputUnit1,
       outputUnit2,
       'OutputUnit.imports $message',
       outputUnit1.imports,
       outputUnit2.imports,
       (a, b) => areElementsEquivalent(a.declaration, b.declaration));
 }