Fix wrong assert from 9180

src/runtime.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2863 matching lines @@
 }
 
 
 template<typename ResultSeqString>
 MUST_USE_RESULT static MaybeObject* StringReplaceStringWithString(
     Isolate* isolate,
     Handle<String> subject,
     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);
   String* pattern =
       String::cast(pattern_regexp->DataAt(JSRegExp::kAtomPatternIndex));
   int subject_len = subject->length();
   int pattern_len = pattern->length();
   int replacement_len = (replacement.is_null()) ? 0 : replacement->length();
 
   FindStringIndicesDispatch(isolate, *subject, pattern, &indices, 0xffffffff);
@@ skipping 10233 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal