Use github mirrors to pull dependencies

Use github mirrors to pull dependencies

There are several benefits of this:
  Devs and bots are pulling from the same location, so issues are easier to track down
  Devs can make sure the mirrors are working locally before rolling deps
  We pull from the same place for both dart-lang and community repos

R=whesse@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/3e38d5abcdc9eb1a7bb35236b5a574a9e8e3a1fe

[Bill Hesse, 2015-05-28 12:15:55]

lgtm

[ricow1, 2015-05-28 12:24:27]

Committed patchset #2 (id:20001) manually as
3e38d5abcdc9eb1a7bb35236b5a574a9e8e3a1fe (presubmit successful).