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

pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
index d788549e624144163cbb427de8089d8083434534..1402b1d87b4c2c9b70de105cac81a7b57b73d854 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
@@ -475,10 +475,6 @@ class ConstantStringifier extends ConstantValueVisitor<String, Null> {
     return '(Null)';
   }
 
-  String visitNonConstant(NonConstantValue constant, _) {
-    return '(NonConstant)';
-  }
-
   String visitInt(IntConstantValue constant, _) {
     return '(Int ${constant.toDartText()})';
   }