contains method Null safety

Whether this string contains a match of other.

Example:

var string = 'Dart strings';
string.contains('D');                     // true
string.contains(RegExp(r'[A-Z]'));    // true

If startIndex is provided, this method matches only at or after that index:

string.contains('D', 1);                  // false
string.contains(RegExp(r'[A-Z]'), 1); // false

The startIndex must not be negative or greater than length.