write a Dart function that counts the frequency of each character (excluding spaces) and prints out

/*Given a string, write a Dart function that counts the frequency of each character (excluding spaces) and prints out the frequency of each unique character*/


void main()
{

  String a1 ="The quickK brown foxX";
  print(charFreqCount(a1));
}

 Map<String,int> charFreqCount(sentence)
 {
  String a2=sentence.replaceAll(' ','');
  List<String> a3=a2.split('');
  print(a3);
  Set<String> s1=Set.from(a3);
  List<String> a4=s1.toList();
  print(a4);

  Map<String,int> abc={};

  for(int i=0;i<a4.length;i++)
  {
    abc[a4[i]]=0;
  }

  for(int i=0;i<a3.length;i++)
  {
    abc[a3[i]]=abc[a3[i]]!+1;
  }

//   print(abc);

   return(abc);

}