`Task.all` should have a canceler that cancels all the tasks passed in

[rorticus]

Calling Task.all(...tasks).cancel() should call cancel on each of those tasks that got passed in.

Something like this might work, but hasn't really been tested, so use at your own risk :)

    static all<T>(iterable: DictionaryOfPromises<T>): Task<{ [key: string]: T }>;
    static all<T>(iterable: (T | Thenable<T>)[]): Task<T[]>;
    static all<T>(iterable: T | Thenable<T>): Task<T[]>;
    static all<T>(iterable: ListOfPromises<T>): Task<T[]>;
    static all<T>(iterable: DictionaryOfPromises<T> | ListOfPromises<T>): Task<any> {
        return new Task<T>((resolve, reject) => {
            super.all(iterable).then(<any> resolve, <any> reject);
        }, () => {
            if (isIterable(iterable) || isArrayLike(iterable)) {
                forOf(iterable, (promiseLike: any) => {
                    if (isTask(promiseLike)) {
                        promiseLike.cancel();
                    }
                });
            }
            else {
                const promiseKeys = Object.keys(iterable);

                promiseKeys.forEach((key) => {
                    const promiseLike = iterable[ key ];

                    if (isTask(promiseLike)) {
                        promiseLike.cancel();
                    }
                });
            }
        });
    }

[dylans]

PR at https://github.com/dojo/core/pull/274

[jacobroufa]

I've made the requested changes to #274, and while the build is failing it is not due to any code related to these set of commits. Please let me know if you would like me to revise anything further!

[kitsonk]

We need to follow through on this.