Signal an error if an Isolate.spawnUri call uses an unsupported URI

Fixes flutter/flutter#76371

Author: jason-simmons

common/settings.h

@@ -118,7 +118,7 @@ struct Settings {
 
   // Used as the script URI in debug messages. Does not affect how the Dart code
   // is executed.
-  std::string advisory_script_uri = "main.dart";
+  std::string advisory_script_uri = "file:///main.dart";
   // Used as the script entrypoint in debug messages. Does not affect how the
   // Dart code is executed.
   std::string advisory_script_entrypoint = "main";

runtime/dart_isolate.cc

@@ -929,6 +929,14 @@ Dart_Isolate DartIsolate::DartIsolateGroupCreateCallback(
   DartIsolateGroupData& parent_group_data =
       (*parent_isolate_data)->GetIsolateGroupData();
 
+  if (parent_group_data.GetSettings().advisory_script_uri !=
+      advisory_script_uri) {
+    std::string error_msg =
+        std::string("Unsupported isolate URI: ") + advisory_script_uri;
+    *error = fml::strdup(error_msg.c_str());
+    return nullptr;
+  }
+
   auto isolate_group_data =
       std::make_unique<std::shared_ptr<DartIsolateGroupData>>(
           std::shared_ptr<DartIsolateGroupData>(new DartIsolateGroupData(