String mapToString(Map m)

Returns a string representing the specified map. The returned string looks like this: '{key0: value0, key1: value1, ... keyN: valueN}'. The value returned by its toString method is used to represent each key or value.

If the map collection contains a reference to itself, either directly as a key or value, or indirectly through other collections or maps, the contained reference is rendered as '{...}'. This prevents the infinite regress that would otherwise occur. So, for example, calling this method on a map whose sole entry maps the string key 'me' to a reference to the map would return '{me: {...}}'.

A typical implementation of a map's toString method will simply return the results of this method applied to the collection.

Source 

static String mapToString(Map m) {
  // Reuse the list in IterableBase for detecting toString cycles.
  if (_isToStringVisiting(m)) {
    return '{...}';
  }

  var result = new StringBuffer();
  try {
    _toStringVisiting.add(m);
    result.write('{');
    bool first = true;
    m.forEach((k, v) {
      if (!first) {
        result.write(', ');
      }
      first = false;
      result.write(k);
      result.write(': ');
      result.write(v);
    });
    result.write('}');
  } finally {
    assert(identical(_toStringVisiting.last, m));
    _toStringVisiting.removeLast();
  }

  return result.toString();
}