It's not a huge issue for me, but a couple more thoughts...  A subclass of `Pair` (e.g. `MutablePair`) could provide a settable value, but that might be useful to Lists as well:

```dart
class Pair {
  final K key;
  V value;
}

/// Add ansi-colors to even and odd lines of cli output.
enumerate(lines).forEach((pair) => pair.value = addColor(pair.key.isEven ? blue : green, pair.value));

// Transform values of specially named keys in a Map.
pairs(map).forEach((pair) => pair.key.endsWith("Handler") && pair.value = handler(pair.value));
```