bool containsValue(Object value)

Returns true if this map contains the given value.

Returns true if any of the values in the map are equal to value according to the == operator.

Source 

bool containsValue(Object value) {
  bool found = false;
  int initialSplayCount = _splayCount;
  bool visit(_SplayTreeMapNode node) {
    while (node != null) {
      if (node.value == value) return true;
      if (initialSplayCount != _splayCount) {
        throw new ConcurrentModificationError(this);
      }
      if (node.right != null && visit(node.right)) return true;
      node = node.left;
    }
    return false;
  }
  return visit(_root);
}