@0x/contracts-erc20-bridge-aggregator: Create package.

`@0x/contracts-erc20`: Add `decimals()` to `LibERC20Token`.

`@0x/contracts-erc20-bridge-sampler`: Created package.

contracts/erc20/contracts/src/LibERC20Token.sol

@@ -24,6 +24,7 @@ import "../src/interfaces/IERC20Token.sol";
 
 
 library LibERC20Token {
+    bytes constant private DECIMALS_CALL_DATA = hex"313ce567";
 
     /// @dev Calls `IERC20Token(token).approve()`.
     ///      Reverts if `false` is returned or if the return
@@ -91,6 +92,21 @@ library LibERC20Token {
         _callWithOptionalBooleanResult(token, callData);
     }
 
+    /// @dev Retrieves the number of decimals for a token.
+    ///      Returns `18` if the call reverts.
+    /// @return The number of decimals places for the token.
+    function decimals(address token)
+        internal
+        view
+        returns (uint8 tokenDecimals)
+    {
+        tokenDecimals = 18;
+        (bool didSucceed, bytes memory resultData) = token.staticcall(DECIMALS_CALL_DATA);
+        if (didSucceed && resultData.length == 32) {
+            tokenDecimals = uint8(LibBytes.readUint256(resultData, 0));
+        }
+    }
+
     /// @dev Executes a call on address `target` with calldata `callData`
     ///      and asserts that either nothing was returned or a single boolean
     ///      was returned equal to `true`.

contracts/erc20/contracts/test/TestLibERC20Token.sol

@@ -69,4 +69,16 @@ contract TestLibERC20Token {
         target.setBehavior(shouldRevert, revertData, returnData);
         LibERC20Token.transferFrom(address(target), from, to, amount);
     }
+
+    function testDecimals(
+        bool shouldRevert,
+        bytes calldata revertData,
+        bytes calldata returnData
+    )
+        external
+        returns (uint8)
+    {
+        target.setBehavior(shouldRevert, revertData, returnData);
+        return LibERC20Token.decimals(address(target));
+    }
 }

contracts/erc20/contracts/test/TestLibERC20TokenTarget.sol

@@ -87,6 +87,14 @@ contract TestLibERC20TokenTarget {
         _execute();
     }
 
+    function decimals()
+        external
+        view
+        returns (uint8)
+    {
+        _execute();
+    }
+
     function _execute() private view {
         if (_shouldRevert) {
             bytes memory revertData = _revertData;