Widen http version constraint.

[pq]

(Required for constraint solving in the SDK build.)

[pq]

@keertip PTAL.

FWIW relevant build failure here:

http://build.chromium.org/p/client.dart/builders/pub-mac-be/builds/13317/steps/annotated_steps/logs/stdio

Salient bit:

Package http has no versions that match ^0.11.2 derived from:
- http_throttle 1.0.0 depends on version >=0.9.0 <0.12.0
- oauth2 0.9.3 depends on version >=0.11.0 <0.12.0
- package_config 0.0.2 depends on version ^0.11.2

[sethladd]

lgtm thanks!

[pq]

Thanks @sethladd !