void prepend(String name, String value, { String separator })

Prepends the value to the previous value of attribute name.

If the attribute was not present, it will just set the attribute name to value. Otherwise, it will use the separator to put between value and the previous value.

If separator is not specified or null, a single space will be used as separator.

Throws an ArgumentError if name or value is null.

Source

void prepend(String name, String value, {String separator});