anchor.diff:     *
      * This constructor iterates over [keys] and [values] and maps each element of
      * [keys] to the corresponding element of [values].
  +   *
  +   *     List<String> letters = ['b', 'c'];
  +   *     List<String> words = ['bad', 'cat'];
  +   *     Map<String, String> map = new Map.fromIterables(letters, words);
  +   *     map['b'] + map['c'];  // badcat
      *
      * If [keys] contains the same object multiple times, the last occurrence
      * overwrites the previous value.
  
positive.code:    * updates the map by mapping [key] to the value returned by
     * [ifAbsent]. Returns the value in the map.
     *
     * It is an error to add or remove keys from the map during the call to
     * [ifAbsent].
     */
    V putIfAbsent(K key, V ifAbsent());
  
  
negative.code: part of dart.core;
  
  /**
   * An unordered collection of key-value pairs, from which you retrieve a value
   * by using its associated key.
   *
   * Each key can occur at most once in a map.
   */
  