Use a trail instead of a mark bit when processing recursive types in the VM

runtime/vm/raw_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 VM_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/token.h"
@@ skipping 1173 matching lines @@
  private:
   RAW_HEAP_OBJECT_IMPLEMENTATION(TypeRef);
 
   RawObject** from() {
     return reinterpret_cast<RawObject**>(&ptr()->type_);
   }
   RawAbstractType* type_;  // The referenced type.
   RawObject** to() {
     return reinterpret_cast<RawObject**>(&ptr()->type_);
   }
-  bool is_being_checked_;  // Transient field, not snapshotted.
 };
 
 
 class RawTypeParameter : public RawAbstractType {
  private:
   RAW_HEAP_OBJECT_IMPLEMENTATION(TypeParameter);
 
   RawObject** from() {
     return reinterpret_cast<RawObject**>(&ptr()->parameterized_class_);
   }
@@ skipping 13 matching lines @@
 
   RawObject** from() {
     return reinterpret_cast<RawObject**>(&ptr()->type_);
   }
   RawAbstractType* type_;
   RawAbstractType* bound_;
   RawTypeParameter* type_parameter_;  // For more detailed error reporting.
   RawObject** to() {
     return reinterpret_cast<RawObject**>(&ptr()->type_parameter_);
   }
-  bool is_being_checked_;  // Transient field, not snapshotted.
 };
 
 
 class RawMixinAppType : public RawAbstractType {
  private:
   RAW_HEAP_OBJECT_IMPLEMENTATION(MixinAppType);
 
   RawObject** from() {
     return reinterpret_cast<RawObject**>(&ptr()->super_type_);
   }
@@ skipping 510 matching lines @@
   // Make sure this is updated when new TypedData types are added.
   ASSERT(kTypedDataInt8ArrayViewCid == kTypedDataInt8ArrayCid + 13);
   ASSERT(kExternalTypedDataInt8ArrayCid == kTypedDataInt8ArrayViewCid + 14);
   ASSERT(kNullCid == kExternalTypedDataInt8ArrayCid + 13);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_