TypeScript: 'extends' is not compiling when used inside declaration file and importHelpers option is switched on

TypeScript Version: 2.1.4 / 2.2.0-dev.20161207

Code

interface Animal {
	type: string;
}

export declare class Pet implements Animal {
	public type: string;
	public name: string;
	constructor();
}

export declare class Dog extends Pet {
	public breed: string;
	constructor();
}

Expected behavior: Compiles successfully.

Actual behavior: Compilation fails with the following error:

error TS2354: This syntax requires an imported helper but module 'tslib' cannot be found.

The same source code compiles fine on TypeScript 2.1.1. Installed tslib package has version 1.2.0.