Added %FlattenString and use it to speed up a regression test.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 70f0d3628f01b1f109f546e85253d04bcb1c8f10..906dfb7bb032ce74c3038f150ae04cab5514ec8a 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -13242,6 +13242,15 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_Abort) {
 }
 
 
+RUNTIME_FUNCTION(MaybeObject*, Runtime_FlattenString) {
+  HandleScope scope(isolate);
+  ASSERT(args.length() == 1);
+  CONVERT_ARG_HANDLE_CHECKED(String, str, 0);
+  FlattenString(str);
+  return isolate->heap()->undefined_value();
+}
+
+
 RUNTIME_FUNCTION(MaybeObject*, Runtime_GetFromCache) {
   // This is only called from codegen, so checks might be more lax.
   CONVERT_ARG_CHECKED(JSFunctionResultCache, cache, 0);