Upgrade dart sdk to 1.13.0-dev.0.0

Upgrade dart sdk to 1.13.0-dev.0.0

My motivation is to be able to use the latest version of the yaml
packages which requires 1.12.0 at minimum (which our previous
pre-release version didn't match).

BUG=
R=zra@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/23cfa759a0c34c706d2e19220d99defa502f5e8d

[tonyg, 2015-09-03 14:30:37]

tonyg@chromium.org changed reviewers:
+ zra@google.com

[zra, 2015-09-03 14:45:29]

https://codereview.chromium.org/1303323005/diff/1/tools/dart/update.py
File tools/dart/update.py (right):

https://codereview.chromium.org/1303323005/diff/1/tools/dart/update.py#newcode20
tools/dart/update.py:20: SDK_URL_BASE =
('http://gsdview.appspot.com/dart-archive/channels/stable/raw/'
It looks like sky is on 1.13.0-dev.0.0:

https://github.com/domokit/sky_engine/blob/master/tools/dart/update.py

would the new yaml package be okay with that? It might be simpler to have both
sky and mojo on the same version.

[tonyg, 2015-09-03 14:49:31]

done -- ptal

[zra, 2015-09-03 14:54:14]

lgtm

[tonyg, 2015-09-03 14:55:07]

Committed patchset #2 (id:20001) manually as
23cfa759a0c34c706d2e19220d99defa502f5e8d (presubmit successful).