.babelrc

// {
//   "presets": [
//     ["@babel/preset-env", { "modules": false }],
//     "@babel/preset-react"
//   ],
//   "plugins": [
//     "@babel/plugin-proposal-class-properties",
//     "@babel/plugin-proposal-object-rest-spread",
//     "@babel/plugin-transform-object-assign",
//     "@babel/plugin-external-helpers"
//   ],
//   "env": {
//     "test": {
//       "plugins": [
//         ["@babel/plugin-transform-runtime", { "runtimeHelpers": true }],
//         "transform-es2015-modules-commonjs"
//       ]
//     }
//   }
// }

{
  "env": {
    "development": {
      "presets": [
        [
          "@babel/preset-env",
          {
            "targets": {
              "browsers": ["> 5%", "Explorer 11"]
            }
          }
        ],
        "@babel/preset-react"
      ],
      "plugins": [
        "@babel/plugin-proposal-class-properties",
        "@babel/plugin-proposal-object-rest-spread",
        "@babel/plugin-transform-object-assign"
        // latest version of rollup-plugin-babel already applies external helpers for us.
        // "@babel/plugin-external-helpers"
      ]
    },
    "production": {
      "presets": [
        [
          "@babel/preset-env",
          {
            "targets": {
              "browsers": ["> 5%", "Explorer 11"]
            }
          }
        ],
        "@babel/preset-react"
      ],
      "plugins": [
        "@babel/plugin-proposal-class-properties",
        "@babel/plugin-proposal-object-rest-spread",
        "@babel/plugin-transform-object-assign"
      ]
    },
    "test": {
      // difference is that we don't use external-helpers
      "presets": [
        [
          "@babel/preset-env",
          {
            "targets": {
              "browsers": ["> 5%", "Explorer 11"],
              "node": "current"
            }
          }
        ],
        "@babel/preset-react"
      ],
      "plugins": [
        "@babel/plugin-proposal-class-properties",
        "@babel/plugin-proposal-object-rest-spread",
        "@babel/plugin-transform-object-assign",
        [
          "@babel/plugin-transform-runtime",
          {
            "regenerator": true
          }
        ]
      ]
    }
  }
}