More work on finalization of recursive types (fixes #29357).

runtime/vm/object.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_OBJECT_H_
 #define RUNTIME_VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 6026 matching lines @@
   static RawType* New(Heap::Space space = Heap::kOld);
 
   FINAL_HEAP_OBJECT_IMPLEMENTATION(Type, AbstractType);
   friend class Class;
   friend class TypeArguments;
   friend class ClearTypeHashVisitor;
 };
 
 
 // A TypeRef is used to break cycles in the representation of recursive types.
-// Its only field is the recursive AbstractType it refers to.
+// Its only field is the recursive AbstractType it refers to, which can
+// temporarily be null during finalization.
 // Note that the cycle always involves type arguments.
 class TypeRef : public AbstractType {
  public:
   virtual bool IsFinalized() const {
-    return AbstractType::Handle(type()).IsFinalized();
+    const AbstractType& ref_type = AbstractType::Handle(type());
+    return !ref_type.IsNull() && ref_type.IsFinalized();
   }
   virtual bool IsBeingFinalized() const {
-    return AbstractType::Handle(type()).IsBeingFinalized();
+    const AbstractType& ref_type = AbstractType::Handle(type());
+    return ref_type.IsNull() || ref_type.IsBeingFinalized();
   }
   virtual bool IsMalformed() const {
     return AbstractType::Handle(type()).IsMalformed();
   }
   virtual bool IsMalbounded() const {
     return AbstractType::Handle(type()).IsMalbounded();
   }
   virtual bool IsMalformedOrMalbounded() const {
     return AbstractType::Handle(type()).IsMalformedOrMalbounded();
   }
@@ skipping 2923 matching lines @@
 
 inline void TypeArguments::SetHash(intptr_t value) const {
   // This is only safe because we create a new Smi, which does not cause
   // heap allocation.
   StoreSmi(&raw_ptr()->hash_, Smi::New(value));
 }
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_OBJECT_H_