Description addDescriptionOf(value)

Appends a description of value. If it is an IMatcher use its describe method; if it is a string use its literal value after escaping any embedded control characters; otherwise use its toString() value and wrap it in angular "quotes".

Source 

Description addDescriptionOf(value) {
  if (value is Matcher) {
    value.describe(this);
  } else {
    add(prettyPrint(value, maxLineLength: 80, maxItems: 25));
  }
  return this;
}