headerIs function

_HeaderMatcher headerIs(
  1. String key,
  2. Matcher value
)

headerIs checks response headers key => value pairs. header can be null or List. So your matcher must be according to Iterable matchers.

eg. your matcher can be:

equals("must-revalidate")

or

contains("must-revalidate")

Implementation

_HeaderMatcher headerIs(String key, Matcher value) =>
    _HeaderMatcher._(key, value);