1. Do not mark an object by storing the object id used during serialization in the object header.

runtime/vm/snapshot_test.cc

Index: runtime/vm/snapshot_test.cc
diff --git a/runtime/vm/snapshot_test.cc b/runtime/vm/snapshot_test.cc
index 6c04f3af2fb524f5a849a9a344f2b84dc7853bb5..51032964c47cb09de1988e8d30c872924cb6bfff 100644
--- a/runtime/vm/snapshot_test.cc
+++ b/runtime/vm/snapshot_test.cc
@@ -255,11 +255,11 @@ Dart_CObject* SerializeAndDeserializeMint(const Mint& mint) {
 
 
 void CheckMint(int64_t value) {
+  ApiNativeScope scope;
   StackZone zone(Thread::Current());
 
   Mint& mint = Mint::Handle();
   mint ^= Integer::New(value);
-  ApiNativeScope scope;
   Dart_CObject* mint_cobject = SerializeAndDeserializeMint(mint);
   // On 64-bit platforms mints always require 64-bits as the smi range
   // here covers most of the 64-bit range. On 32-bit platforms the smi
@@ -476,10 +476,10 @@ Dart_CObject* SerializeAndDeserializeBigint(const Bigint& bigint) {
 
 
 void CheckBigint(const char* bigint_value) {
+  ApiNativeScope scope;
   StackZone zone(Thread::Current());
   Bigint& bigint = Bigint::Handle();
   bigint ^= Bigint::NewFromCString(bigint_value);
-  ApiNativeScope scope;
   Dart_CObject* bigint_cobject = SerializeAndDeserializeBigint(bigint);
   EXPECT_EQ(Dart_CObject_kBigint, bigint_cobject->type);
   char* hex_value = TestCase::BigintToHexValue(bigint_cobject);
@@ -831,6 +831,7 @@ class TestSnapshotWriter : public SnapshotWriter {
   static const intptr_t kInitialSize = 64 * KB;
   TestSnapshotWriter(uint8_t** buffer, ReAlloc alloc)
       : SnapshotWriter(Snapshot::kScript,
+                       Thread::Current(),
                        buffer,
                        alloc,
                        kInitialSize,
@@ -839,7 +840,7 @@ class TestSnapshotWriter : public SnapshotWriter {
                        true, /* can_send_any_object */
                        false, /* snapshot_code */
                        true /* vm_isolate_is_symbolic */),
-        forward_list_(kMaxPredefinedObjectIds) {
+        forward_list_(thread(), kMaxPredefinedObjectIds) {
     ASSERT(buffer != NULL);
     ASSERT(alloc != NULL);
   }
@@ -848,7 +849,6 @@ class TestSnapshotWriter : public SnapshotWriter {
   // Writes just a script object
   void WriteScript(const Script& script) {
     WriteObject(script.raw());
-    UnmarkAll();
   }
 
  private: