Make migration contract not to depend from standard token

contracts/mocks/MigratedERC20Mock.sol

@@ -0,0 +1,24 @@
+pragma solidity ^0.4.24;
+
+import "../token/ERC20/StandardToken.sol";
+import "../token/ERC20/MigratableERC20.sol";
+
+/**
+ * @title MigratedERC20Mock
+ * @dev This contract is a mock to test how a token could be migrated using the MigratableERC20 contract
+ */
+contract MigratedERC20Mock is MigratableERC20, StandardToken {
+
+  constructor(ERC20 _legacyToken) MigratableERC20(_legacyToken) public {}
+
+  /**
+   * @dev Internal minting function
+   * This function will be removed in favour of our new upcoming version of StandardToken
+   */
+  function _mint(address _to, uint256 _amount) internal {
+    require(_to != address(0));
+    totalSupply_ = totalSupply_.add(_amount);
+    balances[_to] = balances[_to].add(_amount);
+    emit Transfer(address(0), _to, _amount);
+  }
+}

contracts/token/ERC20/OptInERC20Migration.sol → contracts/token/ERC20/MigratableERC20.sol

@@ -2,18 +2,17 @@ pragma solidity ^0.4.24;
 
 import "./ERC20.sol";
 import "./SafeERC20.sol";
-import "./StandardToken.sol";
 
 /**
- * @title Opt in ERC20 migration
+ * @title MigratableERC20
  * @dev This strategy carries out an optional migration of the token balances. This migration is performed and paid for
  * @dev by the token holders. The new token contract starts with no initial supply and no balances. The only way to
  * @dev "mint" the new tokens is for users to "turn in" their old ones. This is done by first approving the amount they
  * @dev want to migrate via `ERC20.approve(newTokenAddress, amountToMigrate)` and then calling a function of the new
  * @dev token called `migrateTokens`. The old tokens are sent to a burn address, and the holder receives an equal amount
  * @dev in the new contract.
  */
-contract OptInERC20Migration is StandardToken {
+contract MigratableERC20 {
   using SafeERC20 for ERC20;
 
   /// Burn address where the old tokens are going to be transferred
@@ -56,17 +55,12 @@ contract OptInERC20Migration is StandardToken {
    */
   function migrateTokenTo(address _to, uint256 _amount) public {
     _mint(_to, _amount);
-    legacyToken.transferFrom(msg.sender, BURN_ADDRESS, _amount);
+    legacyToken.safeTransferFrom(msg.sender, BURN_ADDRESS, _amount);
   }
 
   /**
-   * @dev Private minting function
-   * This function will be removed in favour of our new upcoming version of StandardToken
+   * @dev Internal minting function
+   * This function must be overwritten by the implementation
    */
-  function _mint(address _to, uint256 _amount) private {
-    require(_to != address(0));
-    totalSupply_ = totalSupply_.add(_amount);
-    balances[_to] = balances[_to].add(_amount);
-    emit Transfer(address(0), _to, _amount);
-  }
+  function _mint(address _to, uint256 _amount) internal;
 }