[dart2wasm] Add concrete box and symbol for JSBoxedDartObject

The current implementation externalizes and internalizes the Dart
value instead of adding a box and using a runtime-specific symbol.
This makes the implementation consistent with the JS backends.

Change-Id: Iefa382f742bc819b18dfe27ca33741b12473ee39
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/347222
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

CHANGELOG.md

@@ -12,6 +12,15 @@
 
 [pub-security-advisories]: https://dart.dev/go/pub-security-advisories
 
+### Libraries
+
+#### `dart:js_interop`
+
+- On dart2wasm, `JSBoxedDartObject` now is an actual JS object that wraps the
+  opaque Dart value instead of only externalizing the value. Like the JS
+  backends, you'll now get a more useful error when trying to use it in another
+  Dart runtime.
+
 ## 3.3.0
 
 ### Language

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

@@ -94,11 +94,27 @@ extension FunctionToJSExportedDartFunction on Function {
       'by the interop transformer.');
 }
 
+/// Embedded global property for wrapped Dart objects passed via JS interop.
+///
+/// This is a Symbol so that different Dart applications don't share Dart
+/// objects from different Dart runtimes. We expect all [JSBoxedDartObject]s to
+/// have this Symbol.
+final JSSymbol _jsBoxedDartObjectProperty = _boxNonNullable<JSSymbol>(
+    js_helper.JS<WasmExternRef?>('() => Symbol("jsBoxedDartObjectProperty")'));
+
 /// [JSBoxedDartObject] <-> [Object]
 @patch
 extension JSBoxedDartObjectToObject on JSBoxedDartObject {
   @patch
-  Object get toDart => jsObjectToDartObject(toExternRef);
+  Object get toDart {
+    final val = js_helper.JS<WasmExternRef?>('(o,s) => o[s]', this.toExternRef,
+        _jsBoxedDartObjectProperty.toExternRef);
+    if (isDartNull(val)) {
+      throw 'Expected a wrapped Dart object, but got a JS object or a wrapped '
+          'Dart object from a separate runtime instead.';
+    }
+    return jsObjectToDartObject(val);
+  }
 }
 
 @patch
@@ -108,7 +124,10 @@ extension ObjectToJSBoxedDartObject on Object {
     if (this is JSValue) {
       throw 'Attempting to box non-Dart object.';
     }
-    return _boxNonNullable<JSBoxedDartObject>(jsObjectFromDartObject(this));
+    final box = JSObject();
+    js_helper.JS<WasmExternRef?>('(o,s,v) => o[s] = v', box.toExternRef,
+        _jsBoxedDartObjectProperty.toExternRef, jsObjectFromDartObject(this));
+    return box as JSBoxedDartObject;
   }
 }
 

tests/lib/js/static_interop_test/js_types_test.dart

@@ -173,6 +173,7 @@ void syncTests() {
   expect(edo is JSBoxedDartObject, true);
   expect(confuse(edo) is JSBoxedDartObject, true);
   expect(((edo as JSBoxedDartObject).toDart as DartObject).foo, 'bar');
+  expect(edo.instanceOfString('Object'), true);
   // Functions should be boxed without assertInterop.
   final concat = (String a, String b) => a + b;
   edo = concat.toJSBox;