BSD systems don't place bash in /bin and a lot of the dart tools
hardcode a #!/bin/bash shebang that fails the 'all' target build
(not able to execute dart2js since the interpreter is not found).
Solve the issue by using #!/usr/bin/env as the shebang. For scripts
that need to pass arguments to bash modify the script to use the set
command as the first executed line of shell.
BUG=
R=whesse@google.com
Review URL: https://codereview.chromium.org/1552313002 .
William Hesse