class definition for DeferredPromise (why can't they just add it to the ES6 spec?)

deferred-promise.ts

export class DeferredPromise<T> {
    public promise: Promise<T>;
    private resolved: PromiseLike<T>;

    public constructor() {
        this.promise = new Promise((resolve, reject) => {
            if (this.resolved) {
                resolve(this.resolved);
            }
            this.resolve = resolve;
            this.reject = reject;
        });
    }

    public resolve(value?: T | PromiseLike<T>) {
        if (!this.resolved) {
            this.resolved = Promise.resolve(value);
        }
    }

    public reject(reason?: any) {
        if (!this.resolved) {
            this.resolved = Promise.reject(reason);
        }
    }
}