在使用webpack打包过程中，提示以下警告：
WARNING: We noticed you're using the `useBuiltIns` option without declaring a core-js version. Currently, we assume version 2.x when no version is passed.
Since this default
version will likely change in future versions of Babel, we recommend explicitly setting the core-js version you are using via the `corejs` option.
You should also be sure that the version you pass to the `corejs` option matches the version specified in your `package.json`'s `dependencies` section.
If it doesn't, you need to run one of the following commands:
npm install --save core-js@2 npm install --save core-js@3
yarn add core-js@2 yarn add core-js@3
问题解释：警告在使用useBuiltIns选项时，需要声明core-js的版本
解决：在
.babelrc
文件中：
"
presets
"
: [
"
@babel/preset-flow
"
,
"
@babel/preset-env
"
,
"
targets
"
: {
"
node
"
:
"
8.10
"
"
corejs
"
:
"
3
"
,
//
声明corejs版本
"
useBuiltIns
"
:
"
usage
"
