bool contains(Object element)

Returns true if the collection contains an element equal to element.

This operation will check each element in order for being equal to element, unless it has a more efficient way to find an element equal to element.

The equality used to determine whether element is equal to an element of the iterable defaults to the Object.== of the element.

Some types of iterable may have a different equality used for its elements. For example, a Set may have a custom equality (see Set.identical) that its contains uses. Likewise the Iterable returned by a Map.keys call should use the same equality that the Map uses for keys.

Source 

bool contains(Object element) {
  int length = this.length;
  for (int i = 0; i < length; i++) {
    if (elementAt(i) == element) return true;
    if (length != this.length) {
      throw new ConcurrentModificationError(this);
    }
  }
  return false;
}