Revise duplicate interface check

Revise duplicate interface check

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.
Committed: https://code.google.com/p/dart/source/detail?r=20167

[hausner, 2013-03-16 00:55:34]

Welcome back Srdjan :)

[srdjan, 2013-03-18 17:32:24]

lgtm

[hausner, 2013-03-18 17:34:25]

Committed patchset #3 manually as r20167 (presubmit successful).