fix: handle cross-realm Promises in hydratable (#17284)

Author: Rich-Harris

.changeset/olive-mangos-march.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle cross-realm Promises in `hydratable`

packages/svelte/src/internal/server/hydratable.js

@@ -56,7 +56,7 @@ function encode(key, value, unresolved) {
 	let uid = 1;
 
 	entry.serialized = devalue.uneval(entry.value, (value, uneval) => {
-		if (value instanceof Promise) {
+		if (is_promise(value)) {
 			const p = value
 				.then((v) => `r(${uneval(v)})`)
 				.catch((devalue_error) =>
@@ -90,6 +90,16 @@ function encode(key, value, unresolved) {
 	return entry;
 }
 
+/**
+ * @param {any} value
+ * @returns {value is Promise<any>}
+ */
+function is_promise(value) {
+	// we use this check rather than `instanceof Promise`
+	// because it works cross-realm
+	return Object.prototype.toString.call(value) === '[object Promise]';
+}
+
 /**
  * @param {string} key
  * @param {HydratableLookupEntry} a