Turn the NumberToStringStub into a hydrogen stub.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index b27701c4af5513d619156c44926ef2b170ec33eb..060e28276150d4afedad4d4d7a0bd680c2f0ab14 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -941,7 +941,12 @@ class FunctionState V8_FINAL {
 
 class HIfContinuation V8_FINAL {
  public:
-  HIfContinuation() { continuation_captured_ = false; }
+  HIfContinuation() : continuation_captured_(false) {}
+  HIfContinuation(HBasicBlock* true_branch,
+                  HBasicBlock* false_branch,
+                  int position = RelocInfo::kNoPosition)
+      : continuation_captured_(true), true_branch_(true_branch),
+        false_branch_(false_branch), position_(position) {}
   ~HIfContinuation() { ASSERT(!continuation_captured_); }
 
   void Capture(HBasicBlock* true_branch,
@@ -970,6 +975,10 @@ class HIfContinuation V8_FINAL {
     return IsTrueReachable() || IsFalseReachable();
   }
 
+  HBasicBlock* true_branch() const { return true_branch_; }
+  HBasicBlock* false_branch() const { return false_branch_; }
+
+ private:
   bool continuation_captured_;
   HBasicBlock* true_branch_;
   HBasicBlock* false_branch_;
@@ -1228,6 +1237,15 @@ class HGraphBuilder {
                                    ElementsKind to_kind,
                                    bool is_jsarray);
 
+  // Do lookup in the number string cache. If the object is not found
+  // in the cache, the false branch of the continuation is taken;
+  // otherwise the true branch is taken and the returned value contains
+  // the cache value for the object. The returned value must NOT be used
+  // on the false branch.
+  HValue* BuildLookupNumberStringCache(HValue* object,
+                                       HIfContinuation* continuation);
+  HValue* BuildNumberToString(HValue* number);
+
   HInstruction* BuildUncheckedMonomorphicElementAccess(
       HValue* checked_object,
       HValue* key,
@@ -1374,8 +1392,17 @@ class HGraphBuilder {
     void Or();
     void And();
 
+    // Captures the current state of this IfBuilder in the specified
+    // continuation and ends this IfBuilder.
     void CaptureContinuation(HIfContinuation* continuation);
 
+    // Joins the specified continuation from this IfBuilder and ends this
+    // IfBuilder. This appends a Goto instruction from the true branch of
+    // this IfBuilder to the true branch of the continuation unless the
+    // true branch of this IfBuilder is already finished. And vice versa
+    // for the false branch.
+    void JoinContinuation(HIfContinuation* continuation);
+
     void Then();
     void Else();
     void End();