Fix a Debian build error.
Fix a Debian build error.

It turns out it's not safe to assume that pub is checked out from Git
when running from the SDK repo.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1246713005 .

cjkao/dart-pub,dart-lang/pub,computmaxer/pub,maxwellpeterson-wf/pub,dart-lang/pub