anchor.diff:     * 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].
  +   *     Map<String, int> scores = {'Bob': 36};
  +   *     for (var key in ['Bob', 'Rohan', 'Sophena']) {
  +   *       scores.putIfAbsent(key, () => key.length);
  +   *     }
  +   *     scores['Bob'];      // 36
  +   *     scores['Rohan'];    //  5
  +   *     scores['Sophena'];  //  7
  +   *
  +   * The code that [ifAbsent] executes must not add or remove keys.
      */
     V putIfAbsent(K key, V ifAbsent());
   
  
positive.code:   factory Map.identity() = LinkedHashMap<K, V>.identity;
  
    /**
     * Creates a Map instance
     * where the keys and values are computed from the [iterable].
     *
     * For each element of the [iterable] this constructor computes a key-value
     * pair, by applying [key] and [value] respectively.
     *
     * The keys computed by the source [iterable]
     * do not need to be unique. The last
     * occurrence of a key will simply overwrite any previous value.
     *
     * If no values are specified for [key] and [value] the default is the
     * identity function.
     */
    factory Map.fromIterable(Iterable iterable,
        {K key(element), V value(element)}) = LinkedHashMap<K, V>.fromIterable;
  
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.
   */
  