Fix y-umlaut to uppercase.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 9e877ab1582f08b5f7e3b8b4c9c64a3a44f42ca9..5d7d03240d5b47f3969958af1c9afc9552382f1f 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -4098,13 +4098,12 @@ MaybeObject* Heap::LookupSingleCharacterStringFromCode(uint16_t code) {
     return result;
   }
 
-  Object* result;
+  SeqTwoByteString* result;
   { MaybeObject* maybe_result = AllocateRawTwoByteString(1);
-    if (!maybe_result->ToObject(&result)) return maybe_result;
+    if (!maybe_result->To<SeqTwoByteString>(&result)) return maybe_result;
   }
-  String* answer = String::cast(result);
-  answer->Set(0, code);
-  return answer;
+  result->SeqTwoByteStringSet(0, code);
+  return result;
 }