One more tweak for ImportsVerifier - use LibraryElement.exports.
One more tweak for ImportsVerifier - use LibraryElement.exports.

The previous change reduced the imports analysis time from 4.45% to
0.77%, but we can save a little more by using 'exports' directly,
without computing lists of unique exported libraries. We have to break
infinite recursion anyway, and this also takes care of the uniquiness.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org/1894263005 .

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk