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

pkg/compiler/lib/src/js_backend/namer.dart

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 9d86752a40b6e076b2500118701ebfc43a4ff986..c0f21c5a0532fa43ac52357157a34a115b097b02 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -1672,11 +1672,6 @@ class ConstantNamingVisitor implements ConstantValueVisitor {
   }
 
   @override
-  void visitNonConstant(NonConstantValue constant, [_]) {
-    add('null');
-  }
-
-  @override
   void visitInt(IntConstantValue constant, [_]) {
     // No `addRoot` since IntConstants are always inlined.
     if (constant.primitiveValue < 0) {
@@ -1821,9 +1816,6 @@ class ConstantCanonicalHasher implements ConstantValueVisitor<int, Null> {
   int visitNull(NullConstantValue constant, [_]) => 1;
 
   @override
-  int visitNonConstant(NonConstantValue constant, [_]) => 1;
-
-  @override
   int visitBool(BoolConstantValue constant, [_]) {
     return constant.isTrue ? 2 : 3;
   }