Revert "Remove Registry from registerMetadataConstant" and "Serialize metadata"

pkg/compiler/lib/src/constants/values.dart

 // Copyright (c) 2012, 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.constants.values;
 
 import '../common.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart'
@@ skipping 10 matching lines @@
   R visitDouble(DoubleConstantValue constant, A arg);
   R visitBool(BoolConstantValue constant, A arg);
   R visitString(StringConstantValue constant, A arg);
   R visitList(ListConstantValue constant, A arg);
   R visitMap(MapConstantValue constant, A arg);
   R visitConstructed(ConstructedConstantValue constant, A arg);
   R visitType(TypeConstantValue constant, A arg);
   R visitInterceptor(InterceptorConstantValue constant, A arg);
   R visitSynthetic(SyntheticConstantValue constant, A arg);
   R visitDeferred(DeferredConstantValue constant, A arg);
-  R visitNonConstant(NonConstantValue constant, A arg);
 }
 
 abstract class ConstantValue {
   const ConstantValue();
 
   /// `true` if this is a valid constant value.
   bool get isConstant => true;
 
   bool get isNull => false;
   bool get isBool => false;
@@ skipping 714 matching lines @@
 }
 
 /// A constant value resulting from a non constant or erroneous constant
 /// expression.
 // TODO(johnniwinther): Expand this to contain the error kind.
 class NonConstantValue extends ConstantValue {
   bool get isConstant => false;
 
   @override
   accept(ConstantValueVisitor visitor, arg) {
-    return visitor.visitNonConstant(this, arg);
+    // TODO(johnniwinther): Should this be part of the visiting?
   }
 
   @override
   List<ConstantValue> getDependencies() => const <ConstantValue>[];
 
   @override
   DartType getType(CoreTypes types) => const DynamicType();
 
   @override
   String toStructuredText() => 'NonConstant';
 
   @override
   String toDartText() => '>>non-constant<<';
 }