transform contracts to initializers

contracts/mocks/ERC1155BurnableMock.sol

@@ -5,7 +5,9 @@ pragma solidity ^0.5.0;
 import "../token/ERC1155/ERC1155Burnable.sol";
 
 contract ERC1155BurnableMock is ERC1155Burnable {
-    constructor(string memory uri) public ERC1155(uri) { }
+    constructor (string memory uri) public {
+        ERC1155.initialize(uri);
+    }
 
     function mint(address to, uint256 id, uint256 value, bytes memory data) public {
         _mint(to, id, value, data);

contracts/mocks/ERC1155Mock.sol

@@ -9,8 +9,8 @@ import "../token/ERC1155/ERC1155.sol";
  * This mock just publicizes internal functions for testing purposes
  */
 contract ERC1155Mock is ERC1155 {
-    constructor (string memory uri) public ERC1155(uri) {
-        // solhint-disable-previous-line no-empty-blocks
+    constructor (string memory uri) public {
+        ERC1155.initialize(uri);
     }
 
     function setURI(string memory newuri) public {

contracts/token/ERC1155/ERC1155.sol

@@ -2,6 +2,8 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "./IERC1155.sol";
 import "./IERC1155MetadataURI.sol";
 import "./IERC1155Receiver.sol";
@@ -18,7 +20,7 @@ import "../../utils/Address.sol";
  *
  * _Available since v3.1._
  */
-contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
+contract ERC1155 is Initializable, Context, ERC165, IERC1155, IERC1155MetadataURI {
     using SafeMath for uint256;
     using Address for address;
 
@@ -52,7 +54,9 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
     /**
      * @dev See {_setURI}.
      */
-    constructor (string memory uri) public {
+    function initialize(string memory uri) public initializer {
+        ERC165.initialize();
+
         _setURI(uri);
 
         // register the supported interfaces to conform to ERC1155 via ERC165

contracts/token/ERC1155/ERC1155Burnable.sol

@@ -2,6 +2,8 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "./ERC1155.sol";
 
 /**
@@ -10,7 +12,7 @@ import "./ERC1155.sol";
  *
  * _Available since v3.1._
  */
-contract ERC1155Burnable is ERC1155 {
+contract ERC1155Burnable is Initializable, ERC1155 {
     function burn(address account, uint256 id, uint256 value) public {
         require(
             account == _msgSender() || isApprovedForAll(account, _msgSender()),

contracts/token/ERC1155/ERC1155Holder.sol

@@ -2,12 +2,14 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "./ERC1155Receiver.sol";
 
 /**
  * @dev _Available since v3.1._
  */
-contract ERC1155Holder is ERC1155Receiver {
+contract ERC1155Holder is Initializable, ERC1155Receiver {
     function onERC1155Received(address, address, uint256, uint256, bytes memory) public returns (bytes4) {
         return this.onERC1155Received.selector;
     }

contracts/token/ERC1155/ERC1155Receiver.sol

@@ -2,14 +2,18 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "./IERC1155Receiver.sol";
 import "../../introspection/ERC165.sol";
 
 /**
  * @dev _Available since v3.1._
  */
-contract ERC1155Receiver is ERC165, IERC1155Receiver {
-    constructor() public {
+contract ERC1155Receiver is Initializable, ERC165, IERC1155Receiver {
+    function initialize() public initializer {
+        ERC165.initialize();
+
         _registerInterface(
             ERC1155Receiver(0).onERC1155Received.selector ^
             ERC1155Receiver(0).onERC1155BatchReceived.selector

contracts/token/ERC1155/IERC1155.sol

@@ -2,6 +2,8 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "../../introspection/IERC165.sol";
 
 /**
@@ -10,7 +12,7 @@ import "../../introspection/IERC165.sol";
  *
  * _Available since v3.1._
  */
-contract IERC1155 is IERC165 {
+contract IERC1155 is Initializable, IERC165 {
     /**
      * @dev Emitted when `value` tokens of token type `id` are transferred from `from` to `to` by `operator`.
      */

contracts/token/ERC1155/IERC1155MetadataURI.sol

@@ -2,6 +2,8 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "./IERC1155.sol";
 
 /**
@@ -10,7 +12,7 @@ import "./IERC1155.sol";
  *
  * _Available since v3.1._
  */
-contract IERC1155MetadataURI is IERC1155 {
+contract IERC1155MetadataURI is Initializable, IERC1155 {
     /**
      * @dev Returns the URI for token type `id`.
      *

contracts/token/ERC1155/IERC1155Receiver.sol

@@ -2,12 +2,14 @@
 
 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "../../introspection/IERC165.sol";
 
 /**
  * _Available since v3.1._
  */
-contract IERC1155Receiver is IERC165 {
+contract IERC1155Receiver is Initializable, IERC165 {
 
     /**
         @dev Handles the receipt of a single ERC1155 token type. This function is

test/token/ERC1155/ERC1155Holder.test.js

@@ -18,6 +18,7 @@ describe('ERC1155Holder', function () {
   beforeEach(async function () {
     this.multiToken = await ERC1155Mock.new(uri, { from: creator });
     this.holder = await ERC1155Holder.new();
+    await this.holder.initialize();
     await this.multiToken.mintBatch(creator, multiTokenIds, multiTokenAmounts, '0x', { from: creator });
   });