babel/packages/babel-plugin-transform-inline-environment-variables/src/index.js

export default function ({ types: t }) {
  return {
    visitor: {
      MemberExpression(path) {
        if (path.get("object").matchesPattern("process.env")) {
          let key = path.toComputedKey();
          if (t.isStringLiteral(key)) {
            return t.valueToNode(process.env[key.value]);
          }
        }
      }
    }
  };
}