Restore original value of the Dart advisory script URI (flutter#24973)

Some flutter_tester scripts rely on the old value because Dart will convert
"main.dart" into a Platform.script URI containing a fully qualified path to
the script.

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 = "file:///main.dart";
+  std::string advisory_script_uri = "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";