Fix registration of closure functions

Fix registration of closure functions

Closure functions get registered in the class by the
compiler backend. The incorrect assumption was that each
closure only gets visited once. However, closures in finally
clauses are duplicated when the AST nodes of the finally
clause are inlined. This change avoids the duplicate registration.

Fixes issue 18225

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35487

[srdjan, 2014-04-28 17:18:58]

lgtm

[hausner, 2014-04-28 17:20:13]

Committed patchset #1 manually as r35487 (presubmit successful).