React native FileReader.readAsDataURL在react native 0.57中不工作

我需要将从fetch请求接收的图像解析为base64字符串。当我使用RN 0.55.4时，我的代码工作正常，但升级到RN0.57后就不再工作了。 升级后，我记录了我的base64字符串，我得到：

IMAGE64 data:application/octet-stream;base64,77+977+977+977+9AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...(a bunch of "A"s)

下面是代码

    const fileReaderInstance = new FileReader()
       fileReaderInstance.readAsDataURL(response)
        fileReaderInstance.onload = () => {
       //result is always something like -- data:application/octet-
     //stream;base64,77+9UE5HDQoaCgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA//`enter code here`AAAAAAAAAAA... (thousands of As)
             console.log(fileReaderInstance.result)

   }