Throw on spawnUri if we are not on the main thread

Source/bindings/dart/DartController.cpp

Index: Source/bindings/dart/DartController.cpp
diff --git a/Source/bindings/dart/DartController.cpp b/Source/bindings/dart/DartController.cpp
index e816dbe585755b57d9cae02f000a8bdce189e7a5..53d6cbfe48b728bc961b1d48d2d4f954441e878f 100644
--- a/Source/bindings/dart/DartController.cpp
+++ b/Source/bindings/dart/DartController.cpp
@@ -427,6 +427,12 @@ Dart_Isolate DartController::createPureIsolateCallback(const char* scriptURL, co
 {
     bool isSpawnUri = scriptURL ? true : false;
 
+    if (isSpawnUri && !WTF::isMainThread()) {
+        // FIXME(14463): We need to forward this request to the main thread to fetch the URI.
+        *errorMsg = strdup("spawnUri is not yet supported on background isolates.");
+        return 0;
+    }
+
     if (!isSpawnUri) {
         // Determine the parent Isolate's URL as we will be using the same for creating
         // the isolate being spawned using spawnFunction.