[builtins] Migrate String.fromCharCode to TurboFan code stub.

src/runtime/runtime-internal.cc

Index: src/runtime/runtime-internal.cc
diff --git a/src/runtime/runtime-internal.cc b/src/runtime/runtime-internal.cc
index 1bfa6894858cafe5eae22d5c6c2514ff9be29b89..1978e42ff5a4c7e95dc08fa2a52d548fb6c8a207 100644
--- a/src/runtime/runtime-internal.cc
+++ b/src/runtime/runtime-internal.cc
@@ -325,6 +325,25 @@ RUNTIME_FUNCTION(Runtime_AllocateInTargetSpace) {
   return *isolate->factory()->NewFillerObject(size, double_align, space);
 }
 
+RUNTIME_FUNCTION(Runtime_AllocateSeqOneByteString) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_SMI_ARG_CHECKED(length, 0);
+  Handle<SeqOneByteString> result;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, result, isolate->factory()->NewRawOneByteString(length));
+  return *result;
+}
+
+RUNTIME_FUNCTION(Runtime_AllocateSeqTwoByteString) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_SMI_ARG_CHECKED(length, 0);
+  Handle<SeqTwoByteString> result;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, result, isolate->factory()->NewRawTwoByteString(length));
+  return *result;
+}
 
 // Collect the raw data for a stack trace.  Returns an array of 4
 // element segments each containing a receiver, function, code and