Simplified Foreign Bridge for xDai upgrade to only include migrateToMCD functionality. Took the following code as base to apply the changes https://etherscan.io/address/0x0d3726e5a9f37234d6b55216fc971d30f150a60f#code

ForeignBridgeErcToNative.sol

/**
 *Submitted for verification at Etherscan.io on 2018-12-19
*/

pragma solidity 0.4.24;

// File: contracts/ERC677Receiver.sol

contract ERC677Receiver {
    function onTokenTransfer(address _from, uint _value, bytes _data) external returns(bool);
}

// File: openzeppelin-solidity/contracts/token/ERC20/ERC20Basic.sol

/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic {
    function totalSupply() public view returns (uint256);
    function balanceOf(address who) public view returns (uint256);
    function transfer(address to, uint256 value) public returns (bool);
    event Transfer(address indexed from, address indexed to, uint256 value);
}

// File: openzeppelin-solidity/contracts/token/ERC20/ERC20.sol

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
    function allowance(address owner, address spender)
    public view returns (uint256);

    function transferFrom(address from, address to, uint256 value)
    public returns (bool);

    function approve(address spender, uint256 value) public returns (bool);
    event Approval(
        address indexed owner,
        address indexed spender,
        uint256 value
    );
}

// File: contracts/ERC677.sol

contract ERC677 is ERC20 {
    event Transfer(address indexed from, address indexed to, uint value, bytes data);

    function transferAndCall(address, uint, bytes) external returns (bool);

}

// File: contracts/IBurnableMintableERC677Token.sol

contract IBurnableMintableERC677Token is ERC677 {
    function mint(address, uint256) public returns (bool);
    function burn(uint256 _value) public;
    function claimTokens(address _token, address _to) public;
}

// File: contracts/IBridgeValidators.sol

interface IBridgeValidators {
    function isValidator(address _validator) public view returns(bool);
    function requiredSignatures() public view returns(uint256);
    function owner() public view returns(address);
}

// File: contracts/libraries/Message.sol

library Message {
    // function uintToString(uint256 inputValue) internal pure returns (string) {
    //     // figure out the length of the resulting string
    //     uint256 length = 0;
    //     uint256 currentValue = inputValue;
    //     do {
    //         length++;
    //         currentValue /= 10;
    //     } while (currentValue != 0);
    //     // allocate enough memory
    //     bytes memory result = new bytes(length);
    //     // construct the string backwards
    //     uint256 i = length - 1;
    //     currentValue = inputValue;
    //     do {
    //         result[i--] = byte(48 + currentValue % 10);
    //         currentValue /= 10;
    //     } while (currentValue != 0);
    //     return string(result);
    // }

    function addressArrayContains(address[] array, address value) internal pure returns (bool) {
        for (uint256 i = 0; i < array.length; i++) {
            if (array[i] == value) {
                return true;
            }
        }
        return false;
    }
    // layout of message :: bytes:
    // offset  0: 32 bytes :: uint256 - message length
    // offset 32: 20 bytes :: address - recipient address
    // offset 52: 32 bytes :: uint256 - value
    // offset 84: 32 bytes :: bytes32 - transaction hash
    // offset 104: 20 bytes :: address - contract address to prevent double spending

    // bytes 1 to 32 are 0 because message length is stored as little endian.
    // mload always reads 32 bytes.
    // so we can and have to start reading recipient at offset 20 instead of 32.
    // if we were to read at 32 the address would contain part of value and be corrupted.
    // when reading from offset 20 mload will read 12 zero bytes followed
    // by the 20 recipient address bytes and correctly convert it into an address.
    // this saves some storage/gas over the alternative solution
    // which is padding address to 32 bytes and reading recipient at offset 32.
    // for more details see discussion in:
    // https://github.com/paritytech/parity-bridge/issues/61
    function parseMessage(bytes message)
    internal
    pure
    returns(address recipient, uint256 amount, bytes32 txHash, address contractAddress)
    {
        require(isMessageValid(message));
        assembly {
            recipient := and(mload(add(message, 20)), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
            amount := mload(add(message, 52))
            txHash := mload(add(message, 84))
            contractAddress := mload(add(message, 104))
        }
    }

    function isMessageValid(bytes _msg) internal pure returns(bool) {
        return _msg.length == requiredMessageLength();
    }

    function requiredMessageLength() internal pure returns(uint256) {
        return 104;
    }

    function recoverAddressFromSignedMessage(bytes signature, bytes message) internal pure returns (address) {
        require(signature.length == 65);
        bytes32 r;
        bytes32 s;
        bytes1 v;
        // solium-disable-next-line security/no-inline-assembly
        assembly {
            r := mload(add(signature, 0x20))
            s := mload(add(signature, 0x40))
            v := mload(add(signature, 0x60))
        }
        return ecrecover(hashMessage(message), uint8(v), r, s);
    }

    function hashMessage(bytes message) internal pure returns (bytes32) {
        bytes memory prefix = "\x19Ethereum Signed Message:\n";
        // message is always 84 length
        string memory msgLength = "104";
        return keccak256(abi.encodePacked(prefix, msgLength, message));
    }

    function hasEnoughValidSignatures(
        bytes _message,
        uint8[] _vs,
        bytes32[] _rs,
        bytes32[] _ss,
        IBridgeValidators _validatorContract) internal view {
        require(isMessageValid(_message));
        uint256 requiredSignatures = _validatorContract.requiredSignatures();
        require(_vs.length >= requiredSignatures);
        bytes32 hash = hashMessage(_message);
        address[] memory encounteredAddresses = new address[](requiredSignatures);

        for (uint256 i = 0; i < requiredSignatures; i++) {
            address recoveredAddress = ecrecover(hash, _vs[i], _rs[i], _ss[i]);
            require(_validatorContract.isValidator(recoveredAddress));
            if (addressArrayContains(encounteredAddresses, recoveredAddress)) {
                revert();
            }
            encounteredAddresses[i] = recoveredAddress;
        }
    }
}

// File: contracts/libraries/SafeMath.sol

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {

    /**
    * @dev Multiplies two numbers, throws on overflow.
    */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
        assert(c / a == b);
        return c;
    }

    /**
    * @dev Integer division of two numbers, truncating the quotient.
    */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // assert(b > 0); // Solidity automatically throws when dividing by 0
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
        return c;
    }

    /**
    * @dev Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
    */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }

    /**
    * @dev Adds two numbers, throws on overflow.
    */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        assert(c >= a);
        return c;
    }
}

// File: contracts/upgradeability/EternalStorage.sol

/**
 * @title EternalStorage
 * @dev This contract holds all the necessary state variables to carry out the storage of any contract.
 */
contract EternalStorage {

    mapping(bytes32 => uint256) internal uintStorage;
    mapping(bytes32 => string) internal stringStorage;
    mapping(bytes32 => address) internal addressStorage;
    mapping(bytes32 => bytes) internal bytesStorage;
    mapping(bytes32 => bool) internal boolStorage;
    mapping(bytes32 => int256) internal intStorage;

}

// File: contracts/upgradeable_contracts/Ownable.sol

/**
 * @title Ownable
 * @dev This contract has an owner address providing basic authorization control
 */
contract Ownable is EternalStorage {
    /**
    * @dev Event to show ownership has been transferred
    * @param previousOwner representing the address of the previous owner
    * @param newOwner representing the address of the new owner
    */
    event OwnershipTransferred(address previousOwner, address newOwner);

    /**
    * @dev Throws if called by any account other than the owner.
    */
    modifier onlyOwner() {
        require(msg.sender == owner());
        _;
    }

    /**
    * @dev Tells the address of the owner
    * @return the address of the owner
    */
    function owner() public view returns (address) {
        return addressStorage[keccak256(abi.encodePacked("owner"))];
    }

    /**
    * @dev Allows the current owner to transfer control of the contract to a newOwner.
    * @param newOwner the address to transfer ownership to.
    */
    function transferOwnership(address newOwner) public onlyOwner {
        require(newOwner != address(0));
        setOwner(newOwner);
    }

    /**
    * @dev Sets a new owner address
    */
    function setOwner(address newOwner) internal {
        emit OwnershipTransferred(owner(), newOwner);
        addressStorage[keccak256(abi.encodePacked("owner"))] = newOwner;
    }
}

// File: contracts/IOwnedUpgradeabilityProxy.sol

interface IOwnedUpgradeabilityProxy {
    function proxyOwner() public view returns (address);
}

// File: contracts/upgradeable_contracts/OwnedUpgradeability.sol

contract OwnedUpgradeability {

    function upgradeabilityAdmin() public view returns (address) {
        return IOwnedUpgradeabilityProxy(this).proxyOwner();
    }

    // Avoid using onlyProxyOwner name to prevent issues with implementation from proxy contract
    modifier onlyIfOwnerOfProxy() {
        require(msg.sender == upgradeabilityAdmin());
        _;
    }
}

// File: contracts/upgradeable_contracts/Validatable.sol

contract Validatable is EternalStorage {
    function validatorContract() public view returns(IBridgeValidators) {
        return IBridgeValidators(addressStorage[keccak256(abi.encodePacked("validatorContract"))]);
    }

    modifier onlyValidator() {
        require(validatorContract().isValidator(msg.sender));
        _;
    }

    function requiredSignatures() public view returns(uint256) {
        return validatorContract().requiredSignatures();
    }

}

// File: contracts/upgradeable_contracts/BasicBridge.sol

contract BasicBridge is EternalStorage, Validatable, Ownable, OwnedUpgradeability {
    using SafeMath for uint256;

    event GasPriceChanged(uint256 gasPrice);
    event RequiredBlockConfirmationChanged(uint256 requiredBlockConfirmations);
    event DailyLimitChanged(uint256 newLimit);
    event ExecutionDailyLimitChanged(uint256 newLimit);

    function getBridgeInterfacesVersion() public pure returns(uint64 major, uint64 minor, uint64 patch) {
        return (2, 2, 0);
    }

    function setGasPrice(uint256 _gasPrice) public onlyOwner {
        require(_gasPrice > 0);
        uintStorage[keccak256(abi.encodePacked("gasPrice"))] = _gasPrice;
        emit GasPriceChanged(_gasPrice);
    }

    function gasPrice() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("gasPrice"))];
    }

    function setRequiredBlockConfirmations(uint256 _blockConfirmations) public onlyOwner {
        require(_blockConfirmations > 0);
        uintStorage[keccak256(abi.encodePacked("requiredBlockConfirmations"))] = _blockConfirmations;
        emit RequiredBlockConfirmationChanged(_blockConfirmations);
    }

    function requiredBlockConfirmations() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("requiredBlockConfirmations"))];
    }

    function deployedAtBlock() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("deployedAtBlock"))];
    }

    function setTotalSpentPerDay(uint256 _day, uint256 _value) internal {
        uintStorage[keccak256(abi.encodePacked("totalSpentPerDay", _day))] = _value;
    }

    function totalSpentPerDay(uint256 _day) public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("totalSpentPerDay", _day))];
    }

    function setTotalExecutedPerDay(uint256 _day, uint256 _value) internal {
        uintStorage[keccak256(abi.encodePacked("totalExecutedPerDay", _day))] = _value;
    }

    function totalExecutedPerDay(uint256 _day) public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("totalExecutedPerDay", _day))];
    }

    function minPerTx() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("minPerTx"))];
    }

    function maxPerTx() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("maxPerTx"))];
    }

    function executionMaxPerTx() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("executionMaxPerTx"))];
    }

    function setInitialize(bool _status) internal {
        boolStorage[keccak256(abi.encodePacked("isInitialized"))] = _status;
    }

    function isInitialized() public view returns(bool) {
        return boolStorage[keccak256(abi.encodePacked("isInitialized"))];
    }

    function getCurrentDay() public view returns(uint256) {
        return now / 1 days;
    }

    function setDailyLimit(uint256 _dailyLimit) public onlyOwner {
        uintStorage[keccak256(abi.encodePacked("dailyLimit"))] = _dailyLimit;
        emit DailyLimitChanged(_dailyLimit);
    }

    function dailyLimit() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("dailyLimit"))];
    }

    function setExecutionDailyLimit(uint256 _dailyLimit) public onlyOwner {
        uintStorage[keccak256(abi.encodePacked("executionDailyLimit"))] = _dailyLimit;
        emit ExecutionDailyLimitChanged(_dailyLimit);
    }

    function executionDailyLimit() public view returns(uint256) {
        return uintStorage[keccak256(abi.encodePacked("executionDailyLimit"))];
    }

    function setExecutionMaxPerTx(uint256 _maxPerTx) external onlyOwner {
        require(_maxPerTx < executionDailyLimit());
        uintStorage[keccak256(abi.encodePacked("executionMaxPerTx"))] = _maxPerTx;
    }

    function setMaxPerTx(uint256 _maxPerTx) external onlyOwner {
        require(_maxPerTx < dailyLimit());
        uintStorage[keccak256(abi.encodePacked("maxPerTx"))] = _maxPerTx;
    }

    function setMinPerTx(uint256 _minPerTx) external onlyOwner {
        require(_minPerTx < dailyLimit() && _minPerTx < maxPerTx());
        uintStorage[keccak256(abi.encodePacked("minPerTx"))] = _minPerTx;
    }

    function withinLimit(uint256 _amount) public view returns(bool) {
        uint256 nextLimit = totalSpentPerDay(getCurrentDay()).add(_amount);
        return dailyLimit() >= nextLimit && _amount <= maxPerTx() && _amount >= minPerTx();
    }

    function withinExecutionLimit(uint256 _amount) public view returns(bool) {
        uint256 nextLimit = totalExecutedPerDay(getCurrentDay()).add(_amount);
        return executionDailyLimit() >= nextLimit && _amount <= executionMaxPerTx();
    }

    function claimTokens(address _token, address _to) public onlyIfOwnerOfProxy {
        require(_to != address(0));
        if (_token == address(0)) {
            _to.transfer(address(this).balance);
            return;
        }

        ERC20Basic token = ERC20Basic(_token);
        uint256 balance = token.balanceOf(this);
        require(token.transfer(_to, balance));
    }


    function isContract(address _addr) internal view returns (bool)
    {
        uint length;
        assembly { length := extcodesize(_addr) }
        return length > 0;
    }
}

// File: contracts/upgradeable_contracts/BasicForeignBridge.sol

contract BasicForeignBridge is EternalStorage, Validatable {
    using SafeMath for uint256;
    /// triggered when relay of deposit from HomeBridge is complete
    event RelayedMessage(address recipient, uint value, bytes32 transactionHash);
    function executeSignatures(uint8[] vs, bytes32[] rs, bytes32[] ss, bytes message) external {
        Message.hasEnoughValidSignatures(message, vs, rs, ss, validatorContract());
        address recipient;
        uint256 amount;
        bytes32 txHash;
        address contractAddress;
        (recipient, amount, txHash, contractAddress) = Message.parseMessage(message);
        if (messageWithinLimits(amount)) {
            require(contractAddress == address(this));
            require(!relayedMessages(txHash));
            setRelayedMessages(txHash, true);
            require(onExecuteMessage(recipient, amount));
            emit RelayedMessage(recipient, amount, txHash);
        } else {
            onFailedMessage(recipient, amount, txHash);
        }
    }

    function onExecuteMessage(address, uint256) internal returns(bool){
        // has to be defined
    }

    function setRelayedMessages(bytes32 _txHash, bool _status) internal {
        boolStorage[keccak256(abi.encodePacked("relayedMessages", _txHash))] = _status;
    }

    function relayedMessages(bytes32 _txHash) public view returns(bool) {
        return boolStorage[keccak256(abi.encodePacked("relayedMessages", _txHash))];
    }

    function messageWithinLimits(uint256) internal view returns(bool) {
        return true;
    }

    function onFailedMessage(address, uint256, bytes32) internal {
    }
}

interface IScdMcdMigration {
    function swapSaiToDai(uint256 wad) external;
    function daiJoin() external returns (address);
}

interface IDaiAdapter {
    function dai() public returns (address);
}

// File: contracts/upgradeable_contracts/erc20_to_native/ForeignBridgeErcToNative.sol

contract ForeignBridgeErcToNative is BasicBridge, BasicForeignBridge {
    event RelayedMessage(address recipient, uint value, bytes32 transactionHash);
    event TokensSwapped(address indexed from, address indexed to, uint256 value);

    function initialize(
        address _validatorContract,
        address _erc20token,
        uint256 _requiredBlockConfirmations,
        uint256 _gasPrice,
        uint256 _maxPerTx,
        uint256 _homeDailyLimit,
        uint256 _homeMaxPerTx,
        address _owner
    ) public returns(bool) {
        require(!isInitialized());
        require(_validatorContract != address(0) && isContract(_validatorContract));
        require(_requiredBlockConfirmations != 0);
        require(_gasPrice > 0);
        require(_homeMaxPerTx < _homeDailyLimit);
        require(_owner != address(0));
        addressStorage[keccak256(abi.encodePacked("validatorContract"))] = _validatorContract;
        setErc20token(_erc20token);
        uintStorage[keccak256(abi.encodePacked("deployedAtBlock"))] = block.number;
        uintStorage[keccak256(abi.encodePacked("requiredBlockConfirmations"))] = _requiredBlockConfirmations;
        uintStorage[keccak256(abi.encodePacked("gasPrice"))] = _gasPrice;
        uintStorage[keccak256(abi.encodePacked("maxPerTx"))] = _maxPerTx;
        uintStorage[keccak256(abi.encodePacked("executionDailyLimit"))] = _homeDailyLimit;
        uintStorage[keccak256(abi.encodePacked("executionMaxPerTx"))] = _homeMaxPerTx;
        setOwner(_owner);
        setInitialize(true);
        return isInitialized();
    }

    function migrateToMCD(address _migrationContract) external onlyOwner {
        bytes32 storageAddress = 0x3378953eb16363e06fd9ea9701d36ed7285d206d9de7df55b778462d74596a89; // keccak256(abi.encodePacked("migrationToMcdCompleted"))

        require(!boolStorage[storageAddress]);
        require(isContract(_migrationContract));

        uint256 curBalance = erc20token().balanceOf(address(this));
        require(erc20token().approve(_migrationContract, curBalance));
        //It is important to note that this action will cause appearing of `Transfer`
        //event as part of the tokens minting
        IScdMcdMigration(_migrationContract).swapSaiToDai(curBalance);
        address saiContract = erc20token();
        address mcdContract = IDaiAdapter(IScdMcdMigration(_migrationContract).daiJoin()).dai();
        setErc20token(mcdContract);
        require(erc20token().balanceOf(address(this)) == curBalance);

        emit TokensSwapped(saiContract, erc20token(), curBalance);
        boolStorage[storageAddress] = true;
    }

    function upgradeToV220(address _newOwner) public onlyIfOwnerOfProxy {
        require(!boolStorage[keccak256(abi.encodePacked("isUpgradedToV220"))]);
        setOwner(_newOwner);
        uintStorage[keccak256(abi.encodePacked("maxPerTx"))] = 100000 ether;
        uintStorage[keccak256(abi.encodePacked("executionDailyLimit"))] = 1000000 ether;
        emit ExecutionDailyLimitChanged(1000000 ether);
        uintStorage[keccak256(abi.encodePacked("executionMaxPerTx"))] = 100000 ether;
        boolStorage[keccak256("isUpgradedToV220")] = true;
    }

    function getBridgeMode() public pure returns(bytes4 _data) {
        return bytes4(keccak256(abi.encodePacked("erc-to-native-core")));
    }

    function claimTokens(address _token, address _to) public onlyIfOwnerOfProxy {
        require(_token != address(erc20token()));
        super.claimTokens(_token, _to);
    }

    function erc20token() public view returns(ERC20) {
        return ERC20(addressStorage[keccak256(abi.encodePacked("erc20token"))]);
    }

    function onExecuteMessage(address _recipient, uint256 _amount) internal returns(bool) {
        setTotalExecutedPerDay(getCurrentDay(), totalExecutedPerDay(getCurrentDay()).add(_amount));
        return erc20token().transfer(_recipient, _amount);
    }

    function setErc20token(address _token) private {
        require(_token != address(0) && isContract(_token));
        addressStorage[keccak256(abi.encodePacked("erc20token"))] = _token;
    }

    function messageWithinLimits(uint256 _amount) internal view returns(bool) {
        return withinExecutionLimit(_amount);
    }

    function onFailedMessage(address, uint256, bytes32) internal {
        revert();
    }
}