Store constant variable initializers in elements.

tests/compiler/dart2js/serialization_test.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_test;
 
 import 'dart:io';
 import 'memory_compiler.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 422 matching lines @@
           element1.name, element2.name);
     check(element1, element2, 'sourcePosition',
           element1.sourcePosition, element2.sourcePosition);
     checkTypes(
         element1, element2, 'type',
         element1.type, element2.type);
     check(element1, element2, 'isConst',
           element1.isConst, element2.isConst);
     check(element1, element2, 'isFinal',
           element1.isFinal, element2.isFinal);
-    if (element1.isConst) {
-      checkConstants(
-          element1, element2, 'constant',
-          element1.constant, element2.constant);
-    }
+    checkConstants(
+        element1, element2, 'constant',
+        element1.constant, element2.constant);
     check(element1, element2, 'isTopLevel',
           element1.isTopLevel, element2.isTopLevel);
     check(element1, element2, 'isStatic',
           element1.isStatic, element2.isStatic);
     check(element1, element2, 'isInstanceMember',
           element1.isInstanceMember, element2.isInstanceMember);
 
     checkElementIdentities(
         element1, element2, 'library',
         element1.library, element2.library);
@@ skipping 219 matching lines @@
   void visitPrefixElement(PrefixElement element1, PrefixElement element2) {
     check(
         element1, element2, 'isDeferred',
         element1.isDeferred, element2.isDeferred);
     checkElementIdentities(
         element1, element2, 'importedLibrary',
         element1.deferredImport, element2.deferredImport);
     // TODO(johnniwinther): Check members.
   }
 }