Add handle method to ErrorResult.

Add handle method to ErrorResult.

The handle method takes care of checking if the error handler function
accepts two arguements or not, and calles it with error and stackTrace if
possible, otherwise just with error.

I noticed the pattern occuring multiple times when working with error results,
so it would be better to have it handled in one place.

Also changed result file to /// comments.

R=sgjesse@google.com

Committed: https://github.com/dart-lang/async/commit/02ec22851ac73233de02d7cf7d7cfbacd0eaab84

[Lasse Reichstein Nielsen, 2015-07-02 09:18:14]

lrn@google.com changed reviewers:
+ nweiz@google.com, sgjesse@google.com

[Søren Gjesse, 2015-07-02 10:16:55]

lgtm

You did not find and use of it inside the package itself?

[Lasse Reichstein Nielsen, 2015-07-02 12:32:31]

On 2015/07/02 10:16:55, Søren Gjesse wrote:
> lgtm
> 
> You did not find and use of it inside the package itself?

Good point, didn't check. I wanted it for something else I was writing, but
there is probably already places where it can be used. I'll check.

[Lasse Reichstein Nielsen, 2015-07-02 12:36:22]

On 2015/07/02 12:32:31, Lasse Reichstein Nielsen wrote:
> On 2015/07/02 10:16:55, Søren Gjesse wrote:
> > lgtm
> > 
> > You did not find and use of it inside the package itself?
> 
> Good point, didn't check. I wanted it for something else I was writing, but
> there is probably already places where it can be used. I'll check.

But no, it wasn't useful for the existing code.

[Lasse Reichstein Nielsen, 2015-07-03 10:12:00]

Committed patchset #1 (id:1) manually as
02ec22851ac73233de02d7cf7d7cfbacd0eaab84 (presubmit successful).

[nweiz, 2015-07-06 20:19:38]

lgtm