LibSafeDowncast unit tests (#2143)

contracts/staking/contracts/test/TestLibSafeDowncast.sol

@@ -0,0 +1,41 @@
+/*
+
+  Copyright 2019 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 ^0.5.9;
+
+import "../src/libs/LibSafeDowncast.sol";
+
+
+contract TestLibSafeDowncast {
+
+    function downcastToUint96(uint256 a)
+        external
+        pure
+        returns (uint96)
+    {
+        return LibSafeDowncast.downcastToUint96(a);
+    }
+
+    function downcastToUint64(uint256 a)
+        external
+        pure
+        returns (uint64)
+    {
+        return LibSafeDowncast.downcastToUint64(a);
+    }
+}