Converts each element to a String and concatenates the strings.
Iterates through elements of this iterable,
converts each one to a String by calling Object.toString,
and then concatenates the strings, with the
separator string interleaved between the elements.
Source
String join([String separator = ""]) {
if (length == 0) return "";
StringBuffer buffer = new StringBuffer()..writeAll(this, separator);
return buffer.toString();
}