message: provide isFast param to buildPayloadForExit
diff: @@ -145,7 +145,7 @@ export class ERC20 extends POSToken {
  return this.exitUtil.buildPayloadForExit(
  burnTransactionHash,
  eventSignature,
  - false
  + isFast
  ).then(payload => {
  return this.rootChainManager.exit(
  payload, option
  