Fix bug with guarded fields and deserialization.

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
===================================================================
--- runtime/vm/snapshot.cc	(revision 29540)
+++ runtime/vm/snapshot.cc	(working copy)
@@ -150,6 +150,8 @@
       isolate_(isolate),
       cls_(Class::Handle()),
       obj_(Object::Handle()),
+      array_(Array::Handle()),
+      field_(Field::Handle()),
       str_(String::Handle()),
       library_(Library::Handle()),
       type_(AbstractType::Handle()),
@@ -789,9 +791,21 @@
       instance_size = cls_.instance_size();
     }
     intptr_t offset = Object::InstanceSize();
+    intptr_t result_cid = result->GetClassId();
     while (offset < instance_size) {
       obj_ = ReadObjectRef();
       result->SetFieldAtOffset(offset, obj_);
+      if (kind_ == Snapshot::kMessage) {
+        cls_ = isolate()->class_table()->At(result_cid);
+        array_ = cls_.OffsetToField();
+        field_ ^= array_.At(offset / kWordSize);
+        // Entries can be null because offset can be outside of instance fields
+        // due to rounded allocation size.
+        if (!field_.IsNull()) {
+          ASSERT(field_.Offset() == offset);
+          field_.UpdateGuardedCidAndLength(obj_);
+        }
+      }
       offset += kWordSize;
     }
     if (kind_ == Snapshot::kFull) {