replaceFirstChar method
Returns a copy of this string having its first character replaced with the result of the specified transform, or the original string if it's empty. transform - function that takes the first character and returns the result of the transform applied to the character.
Implementation
@pragma('vm:prefer-inline')
@pragma('dart2js:tryInline')
String replaceFirstChar(String Function(String) transform) {
  if (isNotEmpty) {
    return transform(this[0]) + substring(1);
  }
  return this;
}