tilde (~) does not expand in dart executable on mac and linux

[DartBot]

This issue was originally filed by ericarnol...@google.com

---

Steps to reproduce

FAILS:
dart --package-root=~/your_package_root

SUCCEEDS:
dart --package-root=$HOME/your_package_root

Try this:
$ echo --thisworks=$HOME thisworks=/src --thisdoesnt=/src
--thisworks=/Users/ericarnold thisworks=/Users/ericarnold/src --thisdoesnt=~/src

We should probably expand the ~ when we see it in the --param=<value>

[dgrove]

Added Duplicate label.
Marked as being merged into #2930.