in jackson/src/main/java/com/spotify/hamcrest/jackson/IsJsonObject.java [120:125]
public IsJsonObject where(String key, Matcher<? super JsonNode> valueMatcher) {
final LinkedHashMap<String, Matcher<? super JsonNode>> newMap =
new LinkedHashMap<>(entryMatchers);
newMap.put(key, valueMatcher);
return new IsJsonObject(newMap);
}