This is a pure refactoring which extracts the base class for
Devirtualization transformation. It is needed to add a new implementation
of devirtualization later, in addition to the current CHA-based
implementation.
Change-Id: Ifbc6160150a842f5322cf5aea13a7c2180507c96
Reviewed-on: https://dart-review.googlesource.com/30620
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>