Support new packages file in dart 1.12

[evanweible-wf]

The format coverage task takes a --package-root option to discover packages. This fails in projects using dart 1.12 with the --no-package-symlinks flag.

[cbracken]

Will try to get a patch landed for this today!

[abarth]

I'm hitting this issue too in trying to integrate package:coverage with Flutter.

[cbracken]

Fixed by 402b821104e58c0b5e5c583af2e64fb9d21a194c.