Store constant variable initializers in elements.

pkg/compiler/lib/src/serialization/element_serialization.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.elements;
 
 import '../common.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
@@ skipping 380 matching lines @@
     return null;
   }
 
   void serialize(
       FieldElement element, ObjectEncoder encoder, SerializedElementKind kind) {
     encoder.setString(Key.NAME, element.name);
     SerializerUtil.serializePosition(element, encoder);
     encoder.setType(Key.TYPE, element.type);
     encoder.setBool(Key.IS_FINAL, element.isFinal);
     encoder.setBool(Key.IS_CONST, element.isConst);
-    if (element.isConst) {
-      ConstantExpression constant = element.constant;
+    ConstantExpression constant = element.constant;
+    if (constant != null) {
       encoder.setConstant(Key.CONSTANT, constant);
     }
     SerializerUtil.serializeParentRelation(element, encoder);
     if (element is EnumConstantElement) {
       EnumConstantElement enumConstant = element;
       encoder.setInt(Key.INDEX, enumConstant.index);
     }
   }
 }
 
@@ skipping 288 matching lines @@
         return new LocalVariableElementZ(decoder);
       case SerializedElementKind.EXTERNAL_LIBRARY:
       case SerializedElementKind.EXTERNAL_LIBRARY_MEMBER:
       case SerializedElementKind.EXTERNAL_STATIC_MEMBER:
       case SerializedElementKind.EXTERNAL_CONSTRUCTOR:
         break;
     }
     throw new UnsupportedError("Unexpected element kind '${elementKind}.");
   }
 }