[vm] AppJIT snapshot loses ImmutableBit of objects

[dcharkes]

I believe we don't set the ImmutableBit when deserializing from appjit snapshots.

sdk/runtime/vm/app_snapshot.cc

Lines 869 to 883 in 0f1addd

	void Deserializer::InitializeHeader(ObjectPtr raw,
	intptr_t class_id,
	intptr_t size,
	bool is_canonical) {
	ASSERT(Utils::IsAligned(size, kObjectAlignment));
	uword tags = 0;
	tags = UntaggedObject::ClassIdTag::update(class_id, tags);
	tags = UntaggedObject::SizeTag::update(size, tags);
	tags = UntaggedObject::CanonicalBit::update(is_canonical, tags);
	tags = UntaggedObject::AlwaysSetBit::update(true, tags);
	tags = UntaggedObject::NotMarkedBit::update(true, tags);
	tags = UntaggedObject::OldAndNotRememberedBit::update(true, tags);
	tags = UntaggedObject::NewBit::update(false, tags);
	raw->untag()->tags_ = tags;
	}

This leads to objects being copied instead of being shared on SendPort.send.

cc @mkustermann

Edit: Can be exercised with https://dart-review.googlesource.com/c/sdk/+/354902/17..18 and tools/build.py -mdebug create_platform_sdk runtime && tools/test.py -n vm-appjit-linux-debug-x64 lib/isolate/illegal_msg_mirror_test.

[dcharkes]

For the canonical bit, we make a separate cluster.

Maybe we should make separate clusters for the immutable bit as well.

WDYT @rmacnak-google ?

[rmacnak-google]

The immutable bit is mostly of function of the class, so just initializing it like we do for normal allocation should be enough. (There are edge cases with immutable type data, but I think these can't occur at compile/const-time and end up in an app snapshot.) I don't think we need separate clusters until we starting having some instances of a class be immutable and others not.