code: protected String resolveKey(String alias) {
  		List<String> possibleKeys = aliases.get(alias);
  		for (String key : possibleKeys) {
  			if (resolver.containsProperty(key)) {
  				return key;
  			}
  		}
  		return null;
  	}
docs: Find the property key that represents the provided alias that has a value
  (not {@code null}).
  
  @param alias
  the property alias to resolve
  @return the property key or null if all keys represented by the alias
  have no value