Revert "Revert "Started looking for the dart plugin registrant at runtime (#31418)" (#32162)"

This reverts commit ab6b671.

Author: gaaclarke

runtime/dart_isolate.cc

@@ -706,7 +706,9 @@ bool DartIsolate::RunFromLibrary(std::optional<std::string> library_name,
                                ? tonic::ToDart(entrypoint.value().c_str())
                                : tonic::ToDart("main");
 
-  InvokeDartPluginRegistrantIfAvailable(library_handle);
+  if (!FindAndInvokeDartPluginRegistrant()) {
+    InvokeDartPluginRegistrantIfAvailable(library_handle);
+  }
 
   auto user_entrypoint_function =
       ::Dart_GetField(library_handle, entrypoint_handle);