Use UniqueSet<T> and Unique<T> in HCheckMaps and HCheckValue.

src/unique.h

Index: src/unique.h
diff --git a/src/unique.h b/src/unique.h
index 38cc336426fa8403db3241b3fcf1fe360d2a368f..96a5329754a44589f8346d5ce0f0bc23f20adeac 100644
--- a/src/unique.h
+++ b/src/unique.h
@@ -83,29 +83,41 @@ class Unique V8_FINAL {
 
   template <typename U>
   bool operator==(const Unique<U>& other) const {
+    ASSERT(IsInitialized() && other.IsInitialized());
     return raw_address_ == other.raw_address_;
   }
 
   template <typename U>
   bool operator!=(const Unique<U>& other) const {
+    ASSERT(IsInitialized() && other.IsInitialized());
     return raw_address_ != other.raw_address_;
   }
 
   intptr_t Hashcode() const {
+    ASSERT(IsInitialized());
     return reinterpret_cast<intptr_t>(raw_address_);
   }
 
-  bool IsNull() {
+  bool IsNull() const {
+    ASSERT(IsInitialized());
     return raw_address_ == NULL;
   }
 
-  // Don't do this unless you have access to the heap!
-  // No, seriously! You can compare and hash and set-ify uniques that were
-  // all created at the same time; please don't dereference.
-  Handle<T> handle() {
+  // Extract the handle from this Unique in order to dereference it.
+  // WARNING: Only do this if you have access to the heap.
+  Handle<T> handle() const {
     return handle_;
   }
 
+  bool IsInitialized() const {
+    return raw_address_ != NULL || handle_.is_null();
+  }
+
+  // TODO(titzer): this is a hack to migrate to Unique<T> incrementally.
+  static Unique<T> CreateUninitialized(Handle<T> handle) {
+    return Unique<T>(static_cast<Address>(NULL), handle);
+  }
+
   friend class UniqueSet<T>;  // Uses internal details for speed.
   template <class U>
   friend class Unique;  // For comparing raw_address values.
@@ -124,6 +136,7 @@ class UniqueSet V8_FINAL : public ZoneObject {
 
   // Add a new element to this unique set. Mutates this set. O(|this|).
   void Add(Unique<T> uniq, Zone* zone) {
+    ASSERT(uniq.IsInitialized());
     // Keep the set sorted by the {raw_address} of the unique elements.
     for (int i = 0; i < size_; i++) {
       if (array_[i] == uniq) return;