Make arg parser return growable list of rest arguments.
Make arg parser return growable list of rest arguments.

This allows the user to remove them when he's done, which is what
is currently being done.

Review URL: https://codereview.chromium.org//12316155

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@19119 260f80e4-7a28-3924-810f-c04153c831b5

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