CompileType::ToNullableCid must check whether type is implemented in addition to checking whether t…

CompileType::ToNullableCid must check whether type is implemented

This should be done in addition to checking whether type
has subclasses as subtyping relation is defined in terms of interfaces.

Otherwise type propagation infers that result of a mint
operation has a kIntegerCid because IntType has no subclasses.

It is only appropriate to not take subinterfaces into
account when inferring cid of the receiver.

No regression test included because currently it is not possible
to write one: fact that we inferred kIntegerCid for result
of mint operations is not observable and for other IR
instructions nullability prevents us from infering
anything but kDynamicCid.

R=kmillikin@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=19322

[Vyacheslav Egorov (Google), 2013-03-01 13:34:14]

Kevin please take a look.

[Kevin Millikin (Google), 2013-03-01 13:55:03]

LGTM.

[Vyacheslav Egorov (Google), 2013-03-01 16:31:39]

Committed patchset #2 manually as r19322 (presubmit successful).