copy method
Deep copy of the current Chess instance
Implementation
Chess copy() {
return new Chess()
..board = List<Piece>.from(this.board)
..kings = ColorMap<int>.clone(this.kings)
..turn = this.turn
..castling = ColorMap<int>.clone(this.castling)
..epSquare = this.epSquare
..halfMoves = this.halfMoves
..moveNumber = this.moveNumber
..history = List<State>.from(this.history)
..header = Map.from(this.header);
}