Added text/javascript as a possible content-type response to the _validate method

Added text/javascript as a possible content-type response to the _validate method
Some api provider like dropbox core api use text/javascript instead application/json as content-type response
BUG=
R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=33001

[nweiz, 2014-02-24 21:52:01]

Have you signed the contributor license agreement?
https://developers.google.com/open-source/cla/individual?csw=1

[originof, 2014-02-24 22:24:34]

On 2014/02/24 21:52:01, nweiz wrote:
> Have you signed the contributor license agreement?
> https://developers.google.com/open-source/cla/individual?csw=1

Yes

[nweiz, 2014-02-25 01:24:04]

lgtm, I'll merge this in.

[nweiz, 2014-02-25 01:29:19]

Committed patchset #1 manually as r33001 (presubmit successful).