Preserve constant function transition when adding the same function.

src/ia32/stub-cache-ia32.cc

Index: src/ia32/stub-cache-ia32.cc
diff --git a/src/ia32/stub-cache-ia32.cc b/src/ia32/stub-cache-ia32.cc
index 53c9c67ff3aaeea5e3ff5aa6703394017f723397..b2c9dab83bc593ba61230189848880a6875fd479 100644
--- a/src/ia32/stub-cache-ia32.cc
+++ b/src/ia32/stub-cache-ia32.cc
@@ -1571,6 +1571,9 @@ Object* CallStubCompiler::CompileStringCharCodeAtCall(Object* object,
   //  -- esp[(argc + 1) * 4] : receiver
   // -----------------------------------
 
+  // If object is not a string, bail out to regular call.
+  if (!object->IsString()) return Heap::undefined_value();
+
   const int argc = arguments().immediate();
 
   Label miss;
@@ -1581,6 +1584,7 @@ Object* CallStubCompiler::CompileStringCharCodeAtCall(Object* object,
   GenerateDirectLoadGlobalFunctionPrototype(masm(),
                                             Context::STRING_FUNCTION_INDEX,
                                             eax);
+  ASSERT(object != holder);
   CheckPrototypes(JSObject::cast(object->GetPrototype()), eax, holder,
                   ebx, edx, edi, name, &miss);
 
@@ -1635,6 +1639,9 @@ Object* CallStubCompiler::CompileStringCharAtCall(Object* object,
   //  -- esp[(argc + 1) * 4] : receiver
   // -----------------------------------
 
+  // If object is not a string, bail out to regular call.
+  if (!object->IsString()) return Heap::undefined_value();
+
   const int argc = arguments().immediate();
 
   Label miss;
@@ -1646,6 +1653,7 @@ Object* CallStubCompiler::CompileStringCharAtCall(Object* object,
   GenerateDirectLoadGlobalFunctionPrototype(masm(),
                                             Context::STRING_FUNCTION_INDEX,
                                             eax);
+  ASSERT(object != holder);
   CheckPrototypes(JSObject::cast(object->GetPrototype()), eax, holder,
                   ebx, edx, edi, name, &miss);