Prefer Dart_SetReturnValue over Dart_PropagateError when it makes sense.

runtime/bin/file_system_watcher.cc

Index: runtime/bin/file_system_watcher.cc
diff --git a/runtime/bin/file_system_watcher.cc b/runtime/bin/file_system_watcher.cc
index 164346c681ea26389eb40fdc8b9470018c3f24c8..5d8c0b8eaca853bd1b72b45d88b78637dcc84c73 100644
--- a/runtime/bin/file_system_watcher.cc
+++ b/runtime/bin/file_system_watcher.cc
@@ -24,9 +24,7 @@ void FUNCTION_NAME(FileSystemWatcher_InitWatcher)(Dart_NativeArguments args) {
     Dart_SetReturnValue(args, Dart_NewInteger(id));
   } else {
     OSError os_error;
-    Dart_Handle error = DartUtils::NewDartOSError(&os_error);
-    if (Dart_IsError(error)) Dart_PropagateError(error);
-    Dart_ThrowException(error);
+    Dart_ThrowException(DartUtils::NewDartOSError(&os_error));
   }
 }