Added Address.sol for isContract checks to contracts-utils package

contracts/utils/CHANGELOG.json

@@ -13,6 +13,10 @@
             {
                 "note": "Upgrade contracts to Solidity 0.5.5",
                 "pr": 1682
+            },
+            {
+                "note": "Added Address.sol with test for whether or not an address is a contract",
+                "pr": 1657
             }
         ]
     },

contracts/utils/compiler.json

@@ -23,6 +23,7 @@
         }
     },
     "contracts": [
+        "src/Address.sol",
         "src/LibBytes.sol",
         "src/Ownable.sol",
         "src/ReentrancyGuard.sol",

contracts/utils/contracts/src/Address.sol

@@ -0,0 +1,47 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.5.3;
+
+
+/**
+ * Utility library of inline functions on addresses
+ */
+library Address {
+
+    /**
+     * Returns whether the target address is a contract
+     * @dev This function will return false if invoked during the constructor of a contract,
+     * as the code is not actually created until after the constructor finishes.
+     * @param account address of the account to check
+     * @return whether the target address is a contract
+     */
+    function isContract(address account) internal view returns (bool) {
+        uint256 size;
+        // XXX Currently there is no better way to check if there is a contract in an address
+        // than to check the size of the code at that address.
+        // See https://ethereum.stackexchange.com/a/14016/36603
+        // for more details about how this works.
+        // TODO Check this again before the Serenity release, because all addresses will be
+        // contracts then.
+        // solium-disable-next-line security/no-inline-assembly
+        assembly { size := extcodesize(account) }
+        return size > 0;
+    }
+
+}