Create tar files in a cross-platform format.
Create tar files in a cross-platform format.

This passes --format=ustar to the tar process on Linux and OS X, which
should prevent user-specific configuration from causing it to use the
PAX format which is incompatible with 7zip on Windows.

Closes #1411

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1920433002 .

dart-lang/pub,dart-lang/pub