[dart2wasm] Emit encoding for final types.

This marks all types without subtypes as final and all types with
subtypes as non-final. This makes the emitted types maximally final
and is appropriate for a closed-world module.

V8 currently ignores the finality of types for the temporary WasmGC
encoding, which make our current encoding work even though it
implicitly marks some types as final that shouldn't be. For the final
WasmGC instruction encoding, V8 is likely to reject modules containing
final types with subtypes.

Change-Id: If3030c49e8fe60fec8099b731b6fcf618d6a7e64
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310163
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

pkg/wasm_builder/lib/src/types.dart

diff --git a/pkg/wasm_builder/lib/src/types.dart b/pkg/wasm_builder/lib/src/types.dart
index 03524a4..d4b0f52 100644
--- a/pkg/wasm_builder/lib/src/types.dart
+++ b/pkg/wasm_builder/lib/src/types.dart
@@ -602,8 +602,12 @@
   /// The length of the supertype chain of this heap type.
   final int depth;
 
+  bool hasAnySubtypes = false;
+
   DefType({this.superType})
-      : depth = superType != null ? superType.depth + 1 : 0;
+      : depth = superType != null ? superType.depth + 1 : 0 {
+    superType?.hasAnySubtypes = true;
+  }
 
   int get index => _index ?? (throw "$runtimeType $this not added to module");
   set index(int i) => _index = i;
@@ -630,10 +634,13 @@
   // if any.
   void serializeDefinition(Serializer s) {
     if (hasSuperType) {
-      s.writeByte(0x50);
+      s.writeByte(hasAnySubtypes ? 0x50 : 0x4E);
       s.writeUnsigned(1);
       assert(isStructuralSubtypeOf(superType!));
       s.write(superType!);
+    } else if (hasAnySubtypes) {
+      s.writeByte(0x50);
+      s.writeUnsigned(0);
     }
     serializeDefinitionInner(s);
   }