Calculate string hash during flattening.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index be1dbcddd3b64f06e22019a770e5073e32697b70..1b9233f0910a4f151d48ef65b102b2da48dc2b36 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -2380,21 +2380,24 @@ static Object* Runtime_StringBuilderConcat(Arguments args) {
   if (object->IsFailure()) return object;
 
   String* answer = String::cast(object);
+  StringHasher hasher(length);
   for (int i = 0; i < array_length; i++) {
     Object* element = fixed_array->get(i);
     if (element->IsSmi()) {
       int len = Smi::cast(element)->value();
       int pos = len >> 11;
       len &= 0x7ff;
-      String::Flatten(special, answer, pos, pos + len, position);
+      String::Flatten(special, answer, pos, pos + len, position, &hasher);
       position += len;
     } else {
       String* string = String::cast(element);
       int element_length = string->length();
-      String::Flatten(string, answer, 0, element_length, position);
+      String::Flatten(string, answer, 0, element_length, position, &hasher);
       position += element_length;
     }
   }
+  if (hasher.is_valid())
+    answer->set_length_field(hasher.GetHashField());
   return answer;
 }