Implement handlified String::Flatten.

src/jsregexp.cc

Index: src/jsregexp.cc
diff --git a/src/jsregexp.cc b/src/jsregexp.cc
index a30fc26ff0121d4aea7511469f3929625a52eb16..e7184acc1bc4d42c7bbf032d657d152f44e2fe95 100644
--- a/src/jsregexp.cc
+++ b/src/jsregexp.cc
@@ -184,7 +184,7 @@ Handle<Object> RegExpImpl::Compile(Handle<JSRegExp> re,
     re->set_data(*cached);
     return re;
   }
-  pattern = FlattenGetString(pattern);
+  pattern = String::Flatten(pattern);
   PostponeInterruptsScope postpone(isolate);
   RegExpCompileData parse_result;
   FlatStringReader reader(isolate, pattern);
@@ -290,7 +290,7 @@ int RegExpImpl::AtomExecRaw(Handle<JSRegExp> regexp,
   ASSERT(0 <= index);
   ASSERT(index <= subject->length());
 
-  if (!subject->IsFlat()) FlattenString(subject);
+  subject = String::Flatten(subject);
   DisallowHeapAllocation no_gc;  // ensure vectors stay valid
 
   String* needle = String::cast(regexp->DataAt(JSRegExp::kAtomPatternIndex));
@@ -439,7 +439,7 @@ bool RegExpImpl::CompileIrregexp(Handle<JSRegExp> re,
   JSRegExp::Flags flags = re->GetFlags();
 
   Handle<String> pattern(re->Pattern());
-  if (!pattern->IsFlat()) FlattenString(pattern);
+  pattern = String::Flatten(pattern);
   RegExpCompileData compile_data;
   FlatStringReader reader(isolate, pattern);
   if (!RegExpParser::ParseRegExp(&reader, flags.is_multiline(),
@@ -528,7 +528,7 @@ void RegExpImpl::IrregexpInitialize(Handle<JSRegExp> re,
 
 int RegExpImpl::IrregexpPrepare(Handle<JSRegExp> regexp,
                                 Handle<String> subject) {
-  if (!subject->IsFlat()) FlattenString(subject);
+  subject = String::Flatten(subject);
 
   // Check the asciiness of the underlying storage.
   bool is_ascii = subject->IsOneByteRepresentationUnderneath();
@@ -6015,7 +6015,7 @@ RegExpEngine::CompilationResult RegExpEngine::Compile(
   // Sample some characters from the middle of the string.
   static const int kSampleSize = 128;
 
-  FlattenString(sample_subject);
+  sample_subject = String::Flatten(sample_subject);
   int chars_sampled = 0;
   int half_way = (sample_subject->length() - kSampleSize) / 2;
   for (int i = Max(0, half_way);