VM: Fix CHA for Object.

VM: Fix CHA for Object.

Class hierarchy analysis does not track subclasses of Object.
Therefore, it needs to safely assume that any functions has an override.

This fixes test failures when running with --optimization-counter-threshold=5.

R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=27874

[Florian Schneider, 2013-09-25 10:52:28]

@Regis: Why don't we track subclasses of Object with CHA? I did not find any
explanation in the source code.

[Kevin Millikin (Google), 2013-09-25 10:57:02]

LGTM, but why don't we just track them?

[Florian Schneider, 2013-09-25 11:04:59]

Committed patchset #1 manually as r27874 (presubmit successful).

[regis, 2013-09-25 23:02:49]

LGTM

Thanks for fixing the bug.

We decided not to keep track of the subclasses of Object for efficiency reasons,
because every class extends Object.

The drawback is that we have to assume that any function in Object is overridden
(your fix).

I checked with Srdjan and he thinks that most functions of Object are indeed
overridden by some class, mostly from the core lib. So we should not be
penalized too much by this assumption.