Log the sticky error during isolate shutdown (flutter#8742)

The sticky error may be set in cases such as an unhandled asynchronous
exception.  This is similar to the logging done in the Dart command line
embedder's isolate shutdown callback.

runtime/dart_isolate.cc

@@ -791,6 +791,14 @@ void DartIsolate::AddIsolateShutdownCallback(fml::closure closure) {
 }
 
 void DartIsolate::OnShutdownCallback() {
+  {
+    tonic::DartApiScope api_scope;
+    Dart_Handle sticky_error = Dart_GetStickyError();
+    if (!Dart_IsNull(sticky_error) && !Dart_IsFatalError(sticky_error)) {
+      FML_LOG(ERROR) << Dart_GetError(sticky_error);
+    }
+  }
+
   shutdown_callbacks_.clear();
   if (isolate_shutdown_callback_) {
     isolate_shutdown_callback_();