Using SplayTreeMap to order keys

I wrote a code in dart (with vscode) that order a map using SplayTreeMap from 'dart:collection'.

But when inside flutterflow in a Custom Code function show error "The name 'SplayTreeMap' isn't a class."

I tried to import (in custom code function) with no sucess. Receiving the error above.

Has anyone been able to use SplayTreeMap successfully? Or can visualize the solution to this problem.

Here the code part that flutterflow show error:

final sortedMapCrescente = SplayTreeMap<DateTime,String>.from(varMapDateAndString);