Reland r45243.

Reland r45243.

Type and constant propagator should guard against mutable class ids.

Constant strings can be externalized so we can't fold away class checks against
them.

Prevent CP from removing redefinitions of constants with mutable cids.

BUG=

[Vyacheslav Egorov (Google), 2015-04-17 19:54:07]

vegorov@google.com changed reviewers:
+ fschneider@google.com

[Vyacheslav Egorov (Google), 2015-04-17 19:54:07]

PTAL

Change from the original is in the ConstantPropagator::VisitRedefinition.

[Florian Schneider, 2015-04-20 09:33:32]

lgtm

https://codereview.chromium.org/1091353002/diff/1/runtime/vm/constant_propaga...
File runtime/vm/constant_propagator.cc (right):

https://codereview.chromium.org/1091353002/diff/1/runtime/vm/constant_propaga...
runtime/vm/constant_propagator.cc:326:
CheckClassInstr::IsImmutableClassId(value.GetClassId())) {
Works for now. Could we use the knowledge that the cid of a string can only
change from StringCid -> ExternalStringCid? i.e. even though the cid is not
considered constant, it can only be one of two values.

[Vyacheslav Egorov (Google), 2015-04-20 11:02:49]

Landed as https://code.google.com/p/dart/source/detail?r=45275

git cl failed to close this CL automatically