use helper

src/solidity/types/Mapping.js

@@ -19,8 +19,8 @@ class Mapping extends RefType {
         type: this.type
       }
     }
-    var mapSlot = util.toBN(location.slot).toString(16)
-    mapSlot = ethutil.setLengthLeft('0x' + mapSlot, 32).toString('hex')
+    var mapSlot = util.normalizeHex(ethutil.bufferToHex(location.slot))
+    console.log(mapSlot, mappingsPreimages)
     var mappingPreimages = mappingsPreimages[mapSlot]
     var ret = {}
     for (var i in mappingPreimages) {

src/solidity/types/util.js

@@ -10,7 +10,8 @@ module.exports = {
   toBN: toBN,
   add: add,
   extractLocation: extractLocation,
-  removeLocation: removeLocation
+  removeLocation: removeLocation,
+  normalizeHex: normalizeHex
 }
 
 function decodeIntFromHex (value, byteLength, signed) {