Generate sh-compatible binstubs on Unix.

Previously, binstubs used [[ ]], which is a bash-specific construct.

R=rnystrom@google.com
BUG=21854

Review URL: https://codereview.chromium.org//800733003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42308 260f80e4-7a28-3924-810f-c04153c831b5