contains abstract method
Whether singleCharacterString
occurs in this
sequence of characters.
Returns true only if singleCharacterString
is
a string containing a single character
and that character is one of the characters
in this character sequence, and false otherwise.
This behavior is inherited from Iterable<String>
,
which is why it is not Characters-based.
Use containsAll for a method which acts like
String.contains for characters.
Implementation
@override
bool contains(covariant String singleCharacterString);