I have demo project I'm about to compile to ES5 with ES2015 modules enabled and `tslib` used for external TS helpers:

*package.json*

```
{
  "name": "foo",
  "scripts": {
    "build": "tsc"
  },
  "dependencies": {
    "tslib": "^1.9.3"
  },
  "devDependencies": {
    "typescript": "^3.1.3"
  }
}

```

*tsconfig.json*

```
{
  "compilerOptions": {
    "target": "es5",
    "module": "es2015",
    "outDir": "./lib",
    "rootDir": "./src",
    "importHelpers": true,
    "strict": true,
    "experimentalDecorators": true
  }
}

```

*src/index.ts*

```
function a(target: any) {
    return target;
}

@a
export class Foo {}

```

This results in an error:

> 
> src/index.ts:5:1 - error TS2354: This syntax requires an imported helper but module 'tslib' cannot be found.
> 
> 
> 

While `lib/index.js` is correctly compiled:

```
import * as tslib_1 from "tslib";
function a(target) {
    return target;
}
var Foo = /** @class */ (function () {
    function Foo() {
    }
    Foo = tslib_1.__decorate([
        a
    ], Foo);
    return Foo;
}());
export { Foo };

```

How can this problem be solved?