diff --git a/src/objects.h b/src/objects.h
index 03445e8..e966b3d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -613,6 +613,7 @@
 
 class StringStream;
 class ObjectVisitor;
+class Failure;
 
 struct ValueInfo : public Malloced {
   ValueInfo() : type(FIRST_TYPE), ptr(NULL), str(NULL), number(0) { }
@@ -639,6 +640,10 @@
     *obj = reinterpret_cast<Object*>(this);
     return true;
   }
+  inline Failure* ToFailureUnchecked() {
+    ASSERT(IsFailure());
+    return reinterpret_cast<Failure*>(this);
+  }
   inline Object* ToObjectUnchecked() {
     ASSERT(!IsFailure());
     return reinterpret_cast<Object*>(this);
@@ -648,6 +653,13 @@
     return reinterpret_cast<Object*>(this);
   }
 
+  template<typename T>
+  inline bool To(T** obj) {
+    if (IsFailure()) return false;
+    *obj = T::cast(reinterpret_cast<Object*>(this));
+    return true;
+  }
+
 #ifdef OBJECT_PRINT
   // Prints this object with details.
   inline void Print() {
@@ -3712,6 +3724,16 @@
   // [stub cache]: contains stubs compiled for this map.
   DECL_ACCESSORS(code_cache, Object)
 
+  // [prototype transitions]: cache of prototype transitions.
+  // Prototype transition is a transition that happens
+  // when we change object's prototype to a new one.
+  // Cache format:
+  //    0: finger - index of the first free cell in the cache
+  //    1 + 2 * i: prototype
+  //    2 + 2 * i: target map
+  DECL_ACCESSORS(prototype_transitions, FixedArray)
+  inline FixedArray* unchecked_prototype_transitions();
+
   // Lookup in the map's instance descriptors and fill out the result
   // with the given holder if the name is found. The holder may be
   // NULL when this function is used from the compiler.
@@ -3811,6 +3833,12 @@
 
   void TraverseTransitionTree(TraverseCallback callback, void* data);
 
+  static const int kMaxCachedPrototypeTransitions = 256;
+
+  Object* GetPrototypeTransition(Object* prototype);
+
+  MaybeObject* PutPrototypeTransition(Object* prototype, Map* map);
+
   static const int kMaxPreAllocatedPropertyFields = 255;
 
   // Layout description.
@@ -3821,14 +3849,16 @@
   static const int kInstanceDescriptorsOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
-  static const int kPadStart = kCodeCacheOffset + kPointerSize;
+  static const int kPrototypeTransitionsOffset =
+      kCodeCacheOffset + kPointerSize;
+  static const int kPadStart = kPrototypeTransitionsOffset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
   // being continiously allocated.
   static const int kPointerFieldsBeginOffset = Map::kPrototypeOffset;
   static const int kPointerFieldsEndOffset =
-      Map::kCodeCacheOffset + kPointerSize;
+      Map::kPrototypeTransitionsOffset + kPointerSize;
 
   // Byte offsets within kInstanceSizesOffset.
   static const int kInstanceSizeOffset = kInstanceSizesOffset + 0;