isUppercase function
Checks if the string is entirely uppercase.
A string is considered uppercase if it is unchanged when converted to upper case. Strings without cased characters (digits, symbols) and the empty string are considered uppercase.
Example:
isUppercase('HELLO'); // true
isUppercase('HELLO123'); // true
isUppercase('Hello'); // false
Implementation
bool isUppercase(String str) => _isUppercase(str);