This issue was originally filed by andrew.m...@gmail.com

UnmodifiableMapView must implement toString()

+++
class UnmodifiableMapView<K, V> extends DelegatingMap<K, V> {
  String toString() => super.toString();
}
+++

And class 'DelegatingMap' also must implements 'toString()' based on '_baseMap'.

+++
class DelegatingMap<K, V> implements Map<K, V> {
  Map<K, V> _base;

  String toString() => _base.toString();
}
+++