TypeScript: tslib.__extends called on undefined prototype

TypeScript Version: 2.2.1

Code To make things simple, here’s a repo to easily reproduce this: https://github.com/jsayol/typescript-issue-14734

$ git clone https://github.com/jsayol/typescript-issue-14734.git
$ cd typescript-issue-14734
$ npm install
$ npm run build

And load index.html in the browser.

a.ts:

import { B } from './b';

export class A {
  constructor(public prop: string) {
  }

  b(): B {
    return new B(this.prop, 123);
  }

  log() {
    console.log(`Prop: ${this.prop}`);
  }
}

b.ts

import { A } from './a';

export class B extends A {
  constructor(prop: string, public otherStuff: number) {
    super(`B ${prop}`);
  }
}

index.ts

import { A } from './a';
import { B } from './b';

const a = new A('hello');
const b: B = a.b();
b.log();

Expected behavior: Seeing “B hello” logged in the console.

For reference, putting all the code in a single file generates the correct output:

class A {
  constructor(public prop: string) {
  }

  b(): B {
    return new B(this.prop, 123);
  }

  log() {
    console.log(`Prop: ${this.prop}`);
  }
}

class B extends A {
  constructor(prop: string, public otherStuff: number) {
    super(`B ${prop}`);
  }
}

const a = new A('hello');
const b: B = a.b();
b.log();

Actual behavior:

tslib.es6.js:22 Uncaught TypeError: Object prototype may only be an Object or null: undefined
    at setPrototypeOf (<anonymous>)
    at Object.__extends [as a] (tslib.es6.js:22)
    at b.js:4
    at Object.__webpack_exports__.a (b.js:11)
    at __webpack_require__ (bootstrap 4672cf4…:19)
    at Object.defineProperty.value (bootstrap 4672cf4…:65)
    at __webpack_require__ (bootstrap 4672cf4…:19)
    at Object.<anonymous> (bundle.umd.js:108)
    at __webpack_require__ (bootstrap 4672cf4…:19)
    at Object.<anonymous> (bundle.umd.js:321)

This is the generated b.js, where the problem is:

import * as tslib_1 from "tslib";
import { A } from './a';
var B = (function (_super) {
    tslib_1.__extends(B, _super); // <--- Exception thrown here. "B" is undefined.
    function B(prop, otherStuff) {
        var _this = _super.call(this, "B " + prop) || this;
        _this.otherStuff = otherStuff;
        return _this;
    }
    return B;
}(A));
export { B };
//# sourceMappingURL=b.js.map