dart:io

add abstract method

void add(String name, Object value)

Adds a header value. The header named name will have the value value added to its list of values. Some headers are single valued, and for these adding a value will replace the previous value. If the value is of type DateTime a HTTP date format will be applied. If the value is a List each element of the list will be added separately. For all other types the default toString method will be used.

Source

void add(String name, Object value);