[regexp] Properly flatten string during initialization

src/regexp/jsregexp.cc

Index: src/regexp/jsregexp.cc
diff --git a/src/regexp/jsregexp.cc b/src/regexp/jsregexp.cc
index 4e33e4ac7807849d1b137675bb400d88acf9626b..20f5a289dbd8a789d196ffbe695ce16dfdf81616 100644
--- a/src/regexp/jsregexp.cc
+++ b/src/regexp/jsregexp.cc
@@ -135,6 +135,8 @@ static bool HasFewDifferentCharacters(Handle<String> pattern) {
 MaybeHandle<Object> RegExpImpl::Compile(Handle<JSRegExp> re,
                                         Handle<String> pattern,
                                         JSRegExp::Flags flags) {
+  DCHECK(pattern->IsFlat());
+
   Isolate* isolate = re->GetIsolate();
   Zone zone(isolate->allocator(), ZONE_NAME);
   CompilationCache* compilation_cache = isolate->compilation_cache();
@@ -145,7 +147,7 @@ MaybeHandle<Object> RegExpImpl::Compile(Handle<JSRegExp> re,
     re->set_data(*cached);
     return re;
   }
-  pattern = String::Flatten(pattern);
+
   PostponeInterruptsScope postpone(isolate);
   RegExpCompileData parse_result;
   FlatStringReader reader(isolate, pattern);