Add migration contract (#4)

contracts/v0.5/chain/sidechain/SidechainGatewayMigration.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.5.17;
+
+import "../../references/ERC20/IERC20.sol";
+import "./SidechainGatewayStorage.sol";
+
+contract SidechainGatewayMigration is SidechainGatewayStorage {
+  function migrateTo(address[] calldata _tokens, address _to) external onlyAdmin {
+    IERC20 _token;
+    for (uint256 _i; _i < _tokens.length; _i++) {
+      _token = IERC20(_tokens[_i]);
+      _token.transfer(_to, _token.balanceOf(address(this)));
+    }
+  }
+}

contracts/v0.8/migration/BridgeMigration.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/access/Ownable.sol";
+import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+
+interface IGateway {
+  function paused() external returns (bool);
+
+  function pause() external;
+
+  function unpause() external;
+
+  function withdrawTokenFor(
+    uint256 _withdrawalId,
+    address _user,
+    address _token,
+    uint256 _amount,
+    bytes memory _signatures
+  ) external;
+}
+
+interface IGatewayV2 {
+  function receiveEther() external payable;
+}
+
+contract BridgeMigration is Ownable {
+  IGateway public gateway;
+  address public weth;
+
+  constructor(IGateway _gateway, address _weth) {
+    gateway = _gateway;
+    weth = _weth;
+  }
+
+  fallback() external payable {}
+
+  receive() external payable {}
+
+  function pauseGateway() external onlyOwner {
+    gateway.pause();
+  }
+
+  function unpauseGateway() external onlyOwner {
+    gateway.unpause();
+  }
+
+  function migrateAndTransfer(
+    uint256[] calldata _withdrawalIds,
+    address[] calldata _tokens,
+    uint256[] calldata _amounts,
+    bytes[] calldata _signatures,
+    IGatewayV2 _gatewayV2
+  ) external onlyOwner {
+    require(
+      _withdrawalIds.length > 0 &&
+        _withdrawalIds.length == _tokens.length &&
+        _withdrawalIds.length == _amounts.length &&
+        _withdrawalIds.length == _signatures.length,
+      "BridgeMigration: invalid array length"
+    );
+
+    gateway.unpause();
+    for (uint256 _i; _i < _withdrawalIds.length; _i++) {
+      gateway.withdrawTokenFor(_withdrawalIds[_i], msg.sender, _tokens[_i], _amounts[_i], _signatures[_i]);
+    }
+    gateway.pause();
+
+    for (uint256 _i; _i < _tokens.length; _i++) {
+      if (_tokens[_i] == weth) {
+        _gatewayV2.receiveEther{ value: _amounts[_i] }();
+      } else {
+        IERC20(_tokens[_i]).transfer(address(_gatewayV2), _amounts[_i]);
+      }
+    }
+  }
+}