If the repository has third_party/dart-sdk/ unpacked, use that to
format dart files modified in the current patch.

BUG=459376
R=maruel@chromium.org, zra@google.com

Review URL: https://codereview.chromium.org/933383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294179 0039d316-1c4b-4281-b951-d872f2087c98