Add a @MirrorsUsed declaration to pub's utils library.
Add a @MirrorsUsed declaration to pub's utils library.

This will make it easier for the Tavern project to make a
JS-compatible version of pub.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//187413002

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@33348 260f80e4-7a28-3924-810f-c04153c831b5

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