Store constant variable initializers in elements.

pkg/compiler/lib/src/serialization/modelz.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.
 
 /// Implementation of the element model used for deserialiation.
 ///
 /// These classes are created by [ElementDeserializer] triggered by the
 /// [Deserializer].
 
 library dart2js.serialization.modelz;
@@ skipping 1253 matching lines @@
   List<FunctionElement> get nestedClosures => const <FunctionElement>[];
 }
 
 abstract class FieldElementZ extends DeserializedElementZ
     with
         AnalyzableElementMixin,
         AstElementMixin,
         TypedElementMixin,
         MemberElementMixin
     implements FieldElement {
+  bool _isConst;
   ConstantExpression _constant;
 
   FieldElementZ(ObjectDecoder decoder) : super(decoder);
 
   @override
   ElementKind get kind => ElementKind.FIELD;
 
   @override
   accept(ElementVisitor visitor, arg) {
     return visitor.visitFieldElement(this, arg);
   }
 
   @override
   bool get isFinal => _decoder.getBool(Key.IS_FINAL);
 
+  void _ensureConstant() {
+    if (_isConst == null) {
+      _isConst = _decoder.getBool(Key.IS_CONST);
+      _constant = _decoder.getConstant(Key.CONSTANT, isOptional: true);
+    }
+  }
+
   @override
-  bool get isConst => _decoder.getBool(Key.IS_CONST);
+  bool get isConst {
+    _ensureConstant();
+    return _isConst;
+  }
 
   @override
   ConstantExpression get constant {
-    if (isConst && _constant == null) {
-      _constant = _decoder.getConstant(Key.CONSTANT);
-    }
+    _ensureConstant();
     return _constant;
   }
 
   @override
   Expression get initializer => _unsupported('initializer');
 }
 
 class TopLevelFieldElementZ extends FieldElementZ with LibraryMemberMixin {
   TopLevelFieldElementZ(ObjectDecoder decoder) : super(decoder);
 }
@@ skipping 682 matching lines @@
   }
 
   @override
   ElementKind get kind => ElementKind.PREFIX;
 
   @override
   Element lookupLocalMember(String memberName) {
     return _unsupported('lookupLocalMember');
   }
 }