tsconfig-build.json

{
  "angularCompilerOptions": {
    "flatModuleId": "flat_module",
    "flatModuleOutFile": "index.js",
    "skipTemplateCodegen": true
  },

  "compilerOptions": {
    "baseUrl": ".",
    "declaration": true,
    "experimentalDecorators": true,
    "lib": ["es2015", "dom"],
    "moduleResolution": "node",
    "noImplicitAny": true,
    "outDir": "../node_modules/flat_module",
    "rootDir": "",
    "target": "es2015",
    "typeRoots": ["../node_modules/@types"]
 },

 "files": ["public-api.ts"]
}