Throw on spawnUri if we are not on the main thread

Source/bindings/dart/DartController.cpp

 // Copyright (c) 2009, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 409 matching lines @@
     domData->setIsObservatoryFakeDartDOMData(true);
     Dart_ExitIsolate();
     return serviceIsolate;
 }
 
 
 Dart_Isolate DartController::createPureIsolateCallback(const char* scriptURL, const char* entryPoint, void* data, char** errorMsg)
 {
     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.
         DartApiScope apiScope;
         Dart_Handle parentIsolateRootLibrary = Dart_RootLibrary();
         if (Dart_IsError(parentIsolateRootLibrary)) {
             *errorMsg = strdup(Dart_GetError(parentIsolateRootLibrary));
             return 0;
         }
         Dart_Handle parentIsolateURLHandle = Dart_LibraryUrl(parentIsolateRootLibrary);
@@ skipping 492 matching lines @@
     Dart_ExitIsolate();
     loader->processSingleRequest(isolate, url, callback);
 
     // Restore caller isolate.
     Dart_EnterIsolate(caller);
 
     // FIXME: We need some way to return a Dart_Handle to the isolate we just created.
 }
 
 }