Fulfill some code serializer related TODOs.

src/serialize.cc

Index: src/serialize.cc
diff --git a/src/serialize.cc b/src/serialize.cc
index 9d59b6fd4ff51da65939ed77a5c0eba333649ed9..a3d6ebda3bb33cc840d109fad2ab9e586ecfbc29 100644
--- a/src/serialize.cc
+++ b/src/serialize.cc
@@ -1985,6 +1985,7 @@ void CodeSerializer::SerializeCodeStub(Code* stub, HowToCode how_to_code,
          (how_to_code == kFromCode && where_to_point == kInnerPointer));
   uint32_t stub_key = stub->stub_key();
   DCHECK(CodeStub::MajorKeyFromKey(stub_key) != CodeStub::NoCache);
+  DCHECK(CodeStub::GetCode(isolate(), stub_key)->IsCode());
 
   int index = AddCodeStubKey(stub_key) + kCodeStubsBaseIndex;