Move check for duplicate interfaces from parser to class finalizer.
The parser only compared interfaces by name, which did not work
properly for qualified (imported) names. Now the check happens in the
class finalizer after types are finalized.
Sadly, our test harness does not appear to know how to handle multitests
that import libraries.
Review URL: https://codereview.chromium.org//12837007
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@20167 260f80e4-7a28-3924-810f-c04153c831b5
hausner@google.com