[dart2wasm] Simplify boxed Dart object marker allocation

Skip the `JSSymbol` indirection and use an `externref` for the `Symbol`
object directly.

With the `JSSymbol` the indirection cannot be eliminated by inlining,
because we have to check whether the `JSSymbol` static object is
allocated first, and then access its `externref` field.

When using an `externref` directly we still check whether the static
field is initialized, but if it is we directly access the `extenref`,
without going through `JSSymbol`.

Change-Id: I9feae319c34fc5a3f83c0f5ba3767454a1f94566
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445680
Reviewed-by: Srujan Gaddam <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

sdk/lib/_internal/wasm/lib/js_interop_patch.dart

@@ -116,11 +116,8 @@ extension FunctionToJSExportedDartFunction on Function {
 // This is a Symbol so that different Dart applications don't share Dart
 // objects from different Dart runtimes. We expect all JSBoxedDartObjects to
 // have this Symbol.
-final JSSymbol _jsBoxedDartObjectProperty = JSSymbol._(
-  JSValue(
-    js_helper.JS<WasmExternRef?>('() => Symbol("jsBoxedDartObjectProperty")'),
-  ),
-);
+final WasmExternRef? _jsBoxedDartObjectPropertyExternRef = js_helper
+    .JS<WasmExternRef?>('() => Symbol("jsBoxedDartObjectProperty")');
 
 // -----------------------------------------------------------------------------
 // JSBoxedDartObject <-> Object
@@ -131,7 +128,7 @@ extension JSBoxedDartObjectToObject on JSBoxedDartObject {
     final val = js_helper.JS<WasmExternRef?>(
       '(o,s) => o[s]',
       this.toExternRef,
-      _jsBoxedDartObjectProperty.toExternRef,
+      _jsBoxedDartObjectPropertyExternRef,
     );
     if (isDartNull(val)) {
       throw 'Expected a wrapped Dart object, but got a JS object or a wrapped '
@@ -152,7 +149,7 @@ extension ObjectToJSBoxedDartObject on Object {
     js_helper.JS<WasmExternRef?>(
       '(o,s,v) => o[s] = v',
       box.toExternRef,
-      _jsBoxedDartObjectProperty.toExternRef,
+      _jsBoxedDartObjectPropertyExternRef,
       jsObjectFromDartObject(this),
     );
     return JSBoxedDartObject._(box._jsObject);