Fix wrong assert from 9180

There is an optional parameter to the function, with default value being a null handle. We then check that this is a flat string.
Review URL: http://codereview.chromium.org/7850011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9184 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -2881,7 +2881,7 @@ MUST_USE_RESULT static MaybeObject* StringReplaceStringWithString(
     Handle<JSRegExp> pattern_regexp,
     Handle<String> replacement = Handle<String>::null()) {
   ASSERT(subject->IsFlat());
-  ASSERT(replacement->IsFlat());
+  ASSERT(replacement.is_null() || replacement->IsFlat());
 
   ZoneScope zone_space(isolate, DELETE_ON_EXIT);
   ZoneList<int> indices(8);