.babelrc

{
  presets: [
    "@babel/preset-env",
    "@babel/preset-react",
    // The "all" property is needed for Flow to properly parse the syntax. It has
    // trouble understanding when the flow comment is the second comment.
    // See: https://github.com/babel/babel/issues/9845
    ["@babel/preset-flow", { all: true }]
  ],
  plugins: [
    // Without `loose`, the transformation uses `Object.defineProperty` which
    // obeys the spec more, but we get a bundle that's 8kB larger. That was the
    // default in babel v6.
    ["@babel/plugin-proposal-class-properties", { loose: true }],
    ["@babel/plugin-proposal-object-rest-spread", { useBuiltIns: true }],
    "syntax-dynamic-import"
  ],
  env: {
    test: {
      presets: [
        "@babel/preset-env",
        "@babel/preset-react",
        ["@babel/preset-flow", { all: true }]
      ],
      plugins: [
        ["@babel/plugin-proposal-class-properties", { loose: true }],
        ["@babel/plugin-proposal-object-rest-spread", { useBuiltIns: true }],
        "dynamic-import-node"
      ],
    }
  }
}