pub run now defaults to --mode=debug for local executables.

R=rnystrom@google.com
BUG=20898

Review URL: https://codereview.chromium.org//550643003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40384 260f80e4-7a28-3924-810f-c04153c831b5