問題記錄：使用webpack配置中useBuiltIns:'usage'後提示core-js警告

在使用webpack打包過程當中，提示如下警告：

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

問題解釋：警告在使用useBuiltIns選項時，須要聲明core-js的版本

解決：在 .babelrc 文件中：

{
  "presets": [
    "@babel/preset-flow",
    [
      "@babel/preset-env",
      {
        "targets": {
          "node": "8.10"
        },
        "corejs": "3", // 聲明corejs版本
        "useBuiltIns": "usage"
      }
    ]
  ]
}