anchor.diff:        {K key(element), V value(element)}) = HashMap<K, V>.fromIterable;
   
     /**
  -   * Creates a [Map] associating the given [keys] to [values].
  +   * Creates a Map instance associating the given [keys] to [values].
      *
      * This constructor iterates over [keys] and [values] and maps each element of
      * [keys] to the corresponding element of [values].
  
positive.code:       = HashMap<K, V>.fromIterables;
  
    /**
     * Returns whether this map contains the given [value].
     */
    bool containsValue(Object value);
  
    /**
     * Returns whether this map contains the given [key].
     */
    bool containsKey(Object key);
  
    /**
     * Returns the value for the given [key] or null if [key] is not
     * in the map. Because null values are supported, one should either
     * use containsKey to distinguish between an absent key and a null
     * value, or use the [putIfAbsent] method.
     */
    V operator [](Object key);
  
negative.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());
  