anchor.diff:  /**
    * Mixin that overrides mutating map operations with implementations that throw.
    */
  -class _UnmodifiableMapMixin<K, V> implements Map<K, V> {
  +abstract class _UnmodifiableMapMixin<K, V> implements Map<K, V> {
     void operator[]=(K key, V value) {
       throw new UnsupportedError("Cannot modify unmodifiable map");
     }
  
positive.code:  * A more efficient implementation is usually possible by overriding
   * some of the other members as well.
   */
  class MapBase<K, V> implements Map<K, V> {
    MapBase();  // Prevents use as mixin.
  
    Iterable<K> get keys;
  
negative.code:  * A more efficient implementation is usually possible by overriding
   * some of the other members as well.
   */
  abstract class UnmodifiableMapBase<K, V> =
      MapBase<K, V> with _UnmodifiableMapMixin<K, V>;
  
  /**
  