More strong Promise<T> in lib.es6.d.ts

I think it would be good to be able to define the type of error in Promise type. I suggest promise type to be something like this:

interface Promise<T,U> {
    /**
    * Attaches callbacks for the resolution and/or rejection of the Promise.
    * @param onfulfilled The callback to execute when the Promise is resolved.
    * @param onrejected The callback to execute when the Promise is rejected.
    * @returns A Promise for the completion of which ever callback is executed.
    */
    then<TResult>(onfulfilled?: (value: T) => TResult | PromiseLike<TResult>, onrejected?: (reason: U) => TResult | PromiseLike<TResult>): Promise<TResult>;
    then<TResult>(onfulfilled?: (value: T) => TResult | PromiseLike<TResult>, onrejected?: (reason: U) => void): Promise<TResult>;

    /**
     * Attaches a callback for only the rejection of the Promise.
     * @param onrejected The callback to execute when the Promise is rejected.
     * @returns A Promise for the completion of the callback.
     */
    catch(onrejected?: (reason: U) => T | PromiseLike<T>): Promise<T>;
    catch(onrejected?: (reason: U) => void): Promise<T>;

    [Symbol.toStringTag]: string;
}

Or at least have a Promise<T, U> type which extends Promise<T>.

lib.es6.d.ts の Promise 宣言から始め、issue で提案されている 2 パラメーターの Promise<T,U> 形式と比較します。既存の Promise の使用との互換性を含め、提案が then と catch に与える影響を確認します。意図した型付き拒否の設計について合意され、関連する宣言と検証カバレッジに反映されれば完了です。