Fix type propagation for signature classes.

Fix type propagation for signature classes.

After I enabled nullability tracking for checked mode type assertions,
we need to fix type propagation for signature classes: We can't use
the asserted type to infer a cid for those since it may cause unnecessary
deoptimizations.

R=vegorov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39197

[Florian Schneider, 2014-08-13 13:26:05]

This fixes the repeated deoptimization in checked mode.

[Vyacheslav Egorov (Google), 2014-08-13 13:27:28]

I wonder if we want to assert in the IsImplemented() that we don't pass
signature class there.

Or return true from there?

Otherwise LGTM

[Florian Schneider, 2014-08-13 13:45:04]

Committed patchset #1 manually as r39197 (presubmit successful).