diff --git a/runtime/vm/app_snapshot.cc b/runtime/vm/app_snapshot.cc
index c98c358d205..be5d10d224c 100644
--- a/runtime/vm/app_snapshot.cc
+++ b/runtime/vm/app_snapshot.cc
@@ -231,16 +231,19 @@ class DeserializationCluster : public ZoneAllocated {
   virtual void ReadAlloc(Deserializer* deserializer) = 0;
 
   // Initialize the cluster's objects. Do not touch the memory of other objects.
-  virtual void ReadFill(Deserializer* deserializer, bool primary) = 0;
+  virtual void ReadFill(Deserializer* deserializer) = 0;
 
   // Complete any action that requires the full graph to be deserialized, such
   // as rehashing.
-  virtual void PostLoad(Deserializer* deserializer,
-                        const Array& refs,
-                        bool primary) {
-    if (!primary && is_canonical()) {
+  virtual void PostLoad(Deserializer* deserializer, const Array& refs) {
+    // We only need to worry about how canonical values are handled during
+    // deserialization if there may be multiple loading units, which only
+    // happens in the precompiled runtime.
+#if defined(DART_PRECOMPILED_RUNTIME)
+    if (is_canonical()) {
       FATAL("%s needs canonicalization but doesn't define PostLoad", name());
     }
+#endif
   }
 
   const char* name() const { return name_; }
@@ -270,11 +273,7 @@ class SerializationRoots {
 class DeserializationRoots {
  public:
   virtual ~DeserializationRoots() {}
-  // Returns true if these roots are the first snapshot loaded into a heap, and
-  // so can assume any canonical objects don't already exist. Returns false if
-  // some other snapshot may be loaded before these roots, and so written
-  // canonical objects need to run canonicalization during load.
-  virtual bool AddBaseObjects(Deserializer* deserializer) = 0;
+  virtual void AddBaseObjects(Deserializer* deserializer) = 0;
   virtual void ReadRoots(Deserializer* deserializer) = 0;
   virtual void PostLoad(Deserializer* deserializer, const Array& refs) = 0;
 };
@@ -1083,7 +1082,7 @@ class ClassDeserializationCluster : public DeserializationCluster {
   ClassDeserializationCluster() : DeserializationCluster("Class") {}
   ~ClassDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     predefined_start_index_ = d->next_index();
     intptr_t count = d->ReadUnsigned();
     ClassTable* table = d->isolate_group()->class_table();
@@ -1104,7 +1103,7 @@ class ClassDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     for (intptr_t id = predefined_start_index_; id < predefined_stop_index_;
@@ -1478,11 +1477,11 @@ class TypeParametersDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("TypeParameters") {}
   ~TypeParametersDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, TypeParameters::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -1567,7 +1566,7 @@ class TypeArgumentsDeserializationCluster
                                            "TypeArguments") {}
   ~TypeArgumentsDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -1578,10 +1577,10 @@ class TypeArgumentsDeserializationCluster
     BuildCanonicalSetFromLayout(d);
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       TypeArgumentsPtr type_args = static_cast<TypeArgumentsPtr>(d.Ref(id));
       const intptr_t length = d.ReadUnsigned();
@@ -1599,13 +1598,13 @@ class TypeArgumentsDeserializationCluster
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(
           d, refs, Array::Handle(object_store->canonical_type_arguments()));
       object_store->set_canonical_type_arguments(table_);
-    } else if (!primary && is_canonical()) {
+    } else if (!is_root_unit_ && is_canonical()) {
       TypeArguments& type_arg = TypeArguments::Handle(d->zone());
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
         type_arg ^= refs.At(i);
@@ -1662,11 +1661,11 @@ class PatchClassDeserializationCluster : public DeserializationCluster {
   PatchClassDeserializationCluster() : DeserializationCluster("PatchClass") {}
   ~PatchClassDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, PatchClass::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -1882,11 +1881,11 @@ class FunctionDeserializationCluster : public DeserializationCluster {
   FunctionDeserializationCluster() : DeserializationCluster("Function") {}
   ~FunctionDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Function::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -1950,7 +1949,7 @@ class FunctionDeserializationCluster : public DeserializationCluster {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (d->kind() == Snapshot::kFullAOT) {
       Function& func = Function::Handle(d->zone());
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
@@ -2042,11 +2041,11 @@ class ClosureDataDeserializationCluster : public DeserializationCluster {
   ClosureDataDeserializationCluster() : DeserializationCluster("ClosureData") {}
   ~ClosureDataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, ClosureData::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2113,11 +2112,11 @@ class FfiTrampolineDataDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("FfiTrampolineData") {}
   ~FfiTrampolineDataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, FfiTrampolineData::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2224,11 +2223,11 @@ class FieldDeserializationCluster : public DeserializationCluster {
   FieldDeserializationCluster() : DeserializationCluster("Field") {}
   ~FieldDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Field::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2277,7 +2276,7 @@ class FieldDeserializationCluster : public DeserializationCluster {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     Field& field = Field::Handle(d->zone());
     if (!IsolateGroup::Current()->use_field_guards()) {
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
@@ -2365,11 +2364,11 @@ class ScriptDeserializationCluster : public DeserializationCluster {
   ScriptDeserializationCluster() : DeserializationCluster("Script") {}
   ~ScriptDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Script::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2437,11 +2436,11 @@ class LibraryDeserializationCluster : public DeserializationCluster {
   LibraryDeserializationCluster() : DeserializationCluster("Library") {}
   ~LibraryDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Library::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2508,11 +2507,11 @@ class NamespaceDeserializationCluster : public DeserializationCluster {
   NamespaceDeserializationCluster() : DeserializationCluster("Namespace") {}
   ~NamespaceDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Namespace::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2572,11 +2571,11 @@ class KernelProgramInfoDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("KernelProgramInfo") {}
   ~KernelProgramInfoDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, KernelProgramInfo::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -2588,7 +2587,7 @@ class KernelProgramInfoDeserializationCluster : public DeserializationCluster {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     Array& array = Array::Handle(d->zone());
     KernelProgramInfo& info = KernelProgramInfo::Handle(d->zone());
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
@@ -3011,7 +3010,7 @@ class CodeDeserializationCluster : public DeserializationCluster {
   CodeDeserializationCluster() : DeserializationCluster("Code") {}
   ~CodeDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     d->set_code_start_index(start_index_);
     const intptr_t count = d->ReadUnsigned();
@@ -3036,7 +3035,7 @@ class CodeDeserializationCluster : public DeserializationCluster {
     code->untag()->state_bits_ = state_bits;
   }
 
-  void ReadFill(Deserializer* d, bool primary) {
+  void ReadFill(Deserializer* d) override {
     ASSERT(!is_canonical());  // Never canonical.
     ReadFill(d, start_index_, stop_index_, false);
 #if defined(DART_PRECOMPILED_RUNTIME)
@@ -3105,7 +3104,7 @@ class CodeDeserializationCluster : public DeserializationCluster {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     d->EndInstructions();
 
 #if !defined(PRODUCT)
@@ -3237,7 +3236,7 @@ class ObjectPoolDeserializationCluster : public DeserializationCluster {
   ObjectPoolDeserializationCluster() : DeserializationCluster("ObjectPool") {}
   ~ObjectPoolDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3247,7 +3246,7 @@ class ObjectPoolDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -3315,7 +3314,7 @@ class ObjectPoolDeserializationCluster : public DeserializationCluster {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
 #if defined(DART_PRECOMPILED_RUNTIME) &&                                       \
     (!defined(PRODUCT) || defined(FORCE_INCLUDE_DISASSEMBLER))
     if (FLAG_disassemble) {
@@ -3426,7 +3425,7 @@ class PcDescriptorsDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("PcDescriptors") {}
   ~PcDescriptorsDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3436,7 +3435,7 @@ class PcDescriptorsDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -3501,7 +3500,7 @@ class CodeSourceMapDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("CodeSourceMap") {}
   ~CodeSourceMapDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3511,7 +3510,7 @@ class CodeSourceMapDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
@@ -3579,7 +3578,7 @@ class CompressedStackMapsDeserializationCluster
       : DeserializationCluster("CompressedStackMaps") {}
   ~CompressedStackMapsDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3589,7 +3588,7 @@ class CompressedStackMapsDeserializationCluster
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
@@ -3685,16 +3684,16 @@ class RODataSerializationCluster
 class RODataDeserializationCluster
     : public CanonicalSetDeserializationCluster<CanonicalStringSet> {
  public:
-  explicit RODataDeserializationCluster(bool is_canonical,
-                                        bool is_root_unit,
-                                        intptr_t cid)
+  explicit RODataDeserializationCluster(intptr_t cid,
+                                        bool is_canonical,
+                                        bool is_root_unit)
       : CanonicalSetDeserializationCluster(is_canonical,
                                            is_root_unit,
                                            "ROData"),
         cid_(cid) {}
   ~RODataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     intptr_t count = d->ReadUnsigned();
     uint32_t running_offset = 0;
@@ -3709,13 +3708,13 @@ class RODataDeserializationCluster
     }
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     // No-op.
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(d, refs,
@@ -3724,7 +3723,7 @@ class RODataDeserializationCluster
       if (d->isolate_group() == Dart::vm_isolate_group()) {
         Symbols::InitFromSnapshot(d->isolate_group());
       }
-    } else if (!primary && is_canonical()) {
+    } else if (!is_root_unit_ && is_canonical()) {
       FATAL("Cannot recanonicalize RO objects.");
     }
   }
@@ -3794,7 +3793,7 @@ class ExceptionHandlersDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("ExceptionHandlers") {}
   ~ExceptionHandlersDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3804,7 +3803,7 @@ class ExceptionHandlersDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -3886,7 +3885,7 @@ class ContextDeserializationCluster : public DeserializationCluster {
   ContextDeserializationCluster() : DeserializationCluster("Context") {}
   ~ContextDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3896,7 +3895,7 @@ class ContextDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -3966,7 +3965,7 @@ class ContextScopeDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("ContextScope") {}
   ~ContextScopeDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -3976,7 +3975,7 @@ class ContextScopeDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4037,11 +4036,11 @@ class UnlinkedCallDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("UnlinkedCall") {}
   ~UnlinkedCallDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, UnlinkedCall::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4103,11 +4102,11 @@ class ICDataDeserializationCluster : public DeserializationCluster {
   ICDataDeserializationCluster() : DeserializationCluster("ICData") {}
   ~ICDataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, ICData::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4167,11 +4166,11 @@ class MegamorphicCacheDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("MegamorphicCache") {}
   ~MegamorphicCacheDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, MegamorphicCache::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4232,11 +4231,11 @@ class SubtypeTestCacheDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("SubtypeTestCache") {}
   ~SubtypeTestCacheDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, SubtypeTestCache::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4295,11 +4294,11 @@ class LoadingUnitDeserializationCluster : public DeserializationCluster {
   LoadingUnitDeserializationCluster() : DeserializationCluster("LoadingUnit") {}
   ~LoadingUnitDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, LoadingUnit::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4363,11 +4362,11 @@ class LanguageErrorDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("LanguageError") {}
   ~LanguageErrorDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, LanguageError::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4428,11 +4427,11 @@ class UnhandledExceptionDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("UnhandledException") {}
   ~UnhandledExceptionDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, UnhandledException::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4555,13 +4554,17 @@ class InstanceSerializationCluster : public SerializationCluster {
 class AbstractInstanceDeserializationCluster : public DeserializationCluster {
  protected:
   explicit AbstractInstanceDeserializationCluster(const char* name,
-                                                  bool is_canonical)
-      : DeserializationCluster(name, is_canonical) {}
+                                                  bool is_canonical,
+                                                  bool is_root_unit)
+      : DeserializationCluster(name, is_canonical),
+        is_root_unit_(is_root_unit) {}
+
+  const bool is_root_unit_;
 
  public:
 #if defined(DART_PRECOMPILED_RUNTIME)
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
-    if (!primary && is_canonical()) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
+    if (!is_root_unit_ && is_canonical()) {
       SafepointMutexLocker ml(
           d->isolate_group()->constant_canonicalization_mutex());
       Instance& instance = Instance::Handle(d->zone());
@@ -4580,13 +4583,16 @@ class InstanceDeserializationCluster
  public:
   explicit InstanceDeserializationCluster(intptr_t cid,
                                           bool is_canonical,
-                                          bool is_immutable)
-      : AbstractInstanceDeserializationCluster("Instance", is_canonical),
+                                          bool is_immutable,
+                                          bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Instance",
+                                               is_canonical,
+                                               is_root_unit),
         cid_(cid),
         is_immutable_(is_immutable) {}
   ~InstanceDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     next_field_offset_in_words_ = d->Read<int32_t>();
@@ -4599,11 +4605,11 @@ class InstanceDeserializationCluster
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     const intptr_t cid = cid_;
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     const bool is_immutable = is_immutable_;
     intptr_t next_field_offset = next_field_offset_in_words_
                                  << kCompressedWordSizeLog2;
@@ -4692,11 +4698,11 @@ class LibraryPrefixDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("LibraryPrefix") {}
   ~LibraryPrefixDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, LibraryPrefix::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -4804,15 +4810,15 @@ class TypeDeserializationCluster
   }
   ~TypeDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Type::InstanceSize());
     BuildCanonicalSetFromLayout(d);
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       TypePtr type = static_cast<TypePtr>(d.Ref(id));
       Deserializer::InitializeHeader(type, kTypeCid, Type::InstanceSize(),
@@ -4822,13 +4828,13 @@ class TypeDeserializationCluster
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(d, refs,
                          Array::Handle(object_store->canonical_types()));
       object_store->set_canonical_types(table_);
-    } else if (!primary && is_canonical()) {
+    } else if (!is_root_unit_ && is_canonical()) {
       AbstractType& type = AbstractType::Handle(d->zone());
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
         type ^= refs.At(i);
@@ -4918,15 +4924,15 @@ class FunctionTypeDeserializationCluster
                                            "FunctionType") {}
   ~FunctionTypeDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, FunctionType::InstanceSize());
     BuildCanonicalSetFromLayout(d);
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       FunctionTypePtr type = static_cast<FunctionTypePtr>(d.Ref(id));
       Deserializer::InitializeHeader(
@@ -4938,13 +4944,13 @@ class FunctionTypeDeserializationCluster
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(
           d, refs, Array::Handle(object_store->canonical_function_types()));
       object_store->set_canonical_function_types(table_);
-    } else if (!primary && is_canonical()) {
+    } else if (!is_root_unit_ && is_canonical()) {
       AbstractType& type = AbstractType::Handle(d->zone());
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
         type ^= refs.At(i);
@@ -5031,15 +5037,15 @@ class RecordTypeDeserializationCluster
                                            "RecordType") {}
   ~RecordTypeDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, RecordType::InstanceSize());
     BuildCanonicalSetFromLayout(d);
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       RecordTypePtr type = static_cast<RecordTypePtr>(d.Ref(id));
       Deserializer::InitializeHeader(
@@ -5049,13 +5055,13 @@ class RecordTypeDeserializationCluster
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(d, refs,
                          Array::Handle(object_store->canonical_record_types()));
       object_store->set_canonical_record_types(table_);
-    } else if (!primary && is_canonical()) {
+    } else if (!is_root_unit_ && is_canonical()) {
       AbstractType& type = AbstractType::Handle(d->zone());
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
         type ^= refs.At(i);
@@ -5145,15 +5151,15 @@ class TypeParameterDeserializationCluster
                                            "TypeParameter") {}
   ~TypeParameterDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, TypeParameter::InstanceSize());
     BuildCanonicalSetFromLayout(d);
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       TypeParameterPtr type = static_cast<TypeParameterPtr>(d.Ref(id));
       Deserializer::InitializeHeader(type, kTypeParameterCid,
@@ -5166,13 +5172,13 @@ class TypeParameterDeserializationCluster
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(
           d, refs, Array::Handle(object_store->canonical_type_parameters()));
       object_store->set_canonical_type_parameters(table_);
-    } else if (!primary && is_canonical()) {
+    } else if (!is_root_unit_ && is_canonical()) {
       TypeParameter& type_param = TypeParameter::Handle(d->zone());
       for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
         type_param ^= refs.At(i);
@@ -5241,18 +5247,20 @@ class ClosureSerializationCluster : public SerializationCluster {
 class ClosureDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit ClosureDeserializationCluster(bool is_canonical)
-      : AbstractInstanceDeserializationCluster("Closure", is_canonical) {}
+  explicit ClosureDeserializationCluster(bool is_canonical, bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Closure",
+                                               is_canonical,
+                                               is_root_unit) {}
   ~ClosureDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Closure::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       ClosurePtr closure = static_cast<ClosurePtr>(d.Ref(id));
       Deserializer::InitializeHeader(closure, kClosureCid,
@@ -5265,7 +5273,7 @@ class ClosureDeserializationCluster
   }
 
 #if defined(DART_PRECOMPILED_RUNTIME)
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     // We only cache the entry point in bare instructions mode (as we need
     // to load the function anyway otherwise).
     ASSERT(d->kind() == Snapshot::kFullAOT);
@@ -5334,11 +5342,13 @@ class MintSerializationCluster : public SerializationCluster {
 class MintDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit MintDeserializationCluster(bool is_canonical)
-      : AbstractInstanceDeserializationCluster("int", is_canonical) {}
+  explicit MintDeserializationCluster(bool is_canonical, bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("int",
+                                               is_canonical,
+                                               is_root_unit) {}
   ~MintDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     const bool mark_canonical = is_canonical();
@@ -5357,7 +5367,7 @@ class MintDeserializationCluster
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) { Deserializer::Local d(d_); }
+  void ReadFill(Deserializer* d_) override { Deserializer::Local d(d_); }
 };
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
@@ -5401,17 +5411,19 @@ class DoubleSerializationCluster : public SerializationCluster {
 class DoubleDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit DoubleDeserializationCluster(bool is_canonical)
-      : AbstractInstanceDeserializationCluster("double", is_canonical) {}
+  explicit DoubleDeserializationCluster(bool is_canonical, bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("double",
+                                               is_canonical,
+                                               is_root_unit) {}
   ~DoubleDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Double::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       DoublePtr dbl = static_cast<DoublePtr>(d.Ref(id));
       Deserializer::InitializeHeader(dbl, kDoubleCid, Double::InstanceSize(),
@@ -5467,21 +5479,25 @@ class Simd128SerializationCluster : public SerializationCluster {
 class Simd128DeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit Simd128DeserializationCluster(intptr_t cid, bool is_canonical)
-      : AbstractInstanceDeserializationCluster("Simd128", is_canonical),
+  explicit Simd128DeserializationCluster(intptr_t cid,
+                                         bool is_canonical,
+                                         bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Simd128",
+                                               is_canonical,
+                                               is_root_unit),
         cid_(cid) {}
   ~Simd128DeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ASSERT_EQUAL(Int32x4::InstanceSize(), Float32x4::InstanceSize());
     ASSERT_EQUAL(Int32x4::InstanceSize(), Float64x2::InstanceSize());
     ReadAllocFixedSize(d, Int32x4::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
     const intptr_t cid = cid_;
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       ObjectPtr vector = d.Ref(id);
       Deserializer::InitializeHeader(vector, cid, Int32x4::InstanceSize(),
@@ -5541,11 +5557,11 @@ class GrowableObjectArrayDeserializationCluster
       : DeserializationCluster("GrowableObjectArray") {}
   ~GrowableObjectArrayDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, GrowableObjectArray::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
@@ -5610,11 +5626,13 @@ class RecordSerializationCluster : public SerializationCluster {
 class RecordDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit RecordDeserializationCluster(bool is_canonical)
-      : AbstractInstanceDeserializationCluster("Record", is_canonical) {}
+  explicit RecordDeserializationCluster(bool is_canonical, bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Record",
+                                               is_canonical,
+                                               is_root_unit) {}
   ~RecordDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -5624,10 +5642,10 @@ class RecordDeserializationCluster
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
-    const bool stamp_canonical = primary && is_canonical();
+    const bool stamp_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       RecordPtr record = static_cast<RecordPtr>(d.Ref(id));
       const intptr_t shape = d.ReadUnsigned();
@@ -5694,7 +5712,7 @@ class TypedDataDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("TypedData"), cid_(cid) {}
   ~TypedDataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     intptr_t element_size = TypedData::ElementSizeInBytes(cid_);
@@ -5705,7 +5723,7 @@ class TypedDataDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -5774,11 +5792,11 @@ class TypedDataViewDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("TypedDataView"), cid_(cid) {}
   ~TypedDataViewDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, TypedDataView::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     const intptr_t cid = cid_;
@@ -5790,8 +5808,7 @@ class TypedDataViewDeserializationCluster : public DeserializationCluster {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
-    ASSERT(primary || !is_canonical());
+  void PostLoad(Deserializer* d, const Array& refs) override {
     auto& view = TypedDataView::Handle(d->zone());
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       view ^= refs.At(id);
@@ -5852,11 +5869,11 @@ class ExternalTypedDataDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("ExternalTypedData"), cid_(cid) {}
   ~ExternalTypedDataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, ExternalTypedData::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -5949,7 +5966,7 @@ class DeltaEncodedTypedDataDeserializationCluster
       : DeserializationCluster("DeltaEncodedTypedData") {}
   ~DeltaEncodedTypedDataDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -5959,7 +5976,7 @@ class DeltaEncodedTypedDataDeserializationCluster
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
     TypedData& typed_data = TypedData::Handle(d_->zone());
 
@@ -6035,11 +6052,11 @@ class StackTraceDeserializationCluster : public DeserializationCluster {
   StackTraceDeserializationCluster() : DeserializationCluster("StackTrace") {}
   ~StackTraceDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, StackTrace::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -6098,11 +6115,11 @@ class RegExpDeserializationCluster : public DeserializationCluster {
   RegExpDeserializationCluster() : DeserializationCluster("RegExp") {}
   ~RegExpDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, RegExp::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -6179,11 +6196,11 @@ class WeakPropertyDeserializationCluster : public DeserializationCluster {
       : DeserializationCluster("WeakProperty") {}
   ~WeakPropertyDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, WeakProperty::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     ASSERT(!is_canonical());  // Never canonical.
@@ -6242,20 +6259,24 @@ class MapSerializationCluster : public SerializationCluster {
 class MapDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit MapDeserializationCluster(bool is_canonical, intptr_t cid)
-      : AbstractInstanceDeserializationCluster("Map", is_canonical),
+  explicit MapDeserializationCluster(intptr_t cid,
+                                     bool is_canonical,
+                                     bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Map",
+                                               is_canonical,
+                                               is_root_unit),
         cid_(cid) {}
   ~MapDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Map::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     const intptr_t cid = cid_;
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       MapPtr map = static_cast<MapPtr>(d.Ref(id));
       Deserializer::InitializeHeader(map, cid, Map::InstanceSize(),
@@ -6313,20 +6334,24 @@ class SetSerializationCluster : public SerializationCluster {
 class SetDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit SetDeserializationCluster(bool is_canonical, intptr_t cid)
-      : AbstractInstanceDeserializationCluster("Set", is_canonical),
+  explicit SetDeserializationCluster(intptr_t cid,
+                                     bool is_canonical,
+                                     bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Set",
+                                               is_canonical,
+                                               is_root_unit),
         cid_(cid) {}
   ~SetDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     ReadAllocFixedSize(d, Set::InstanceSize());
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     const intptr_t cid = cid_;
-    const bool mark_canonical = primary && is_canonical();
+    const bool mark_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       SetPtr set = static_cast<SetPtr>(d.Ref(id));
       Deserializer::InitializeHeader(set, cid, Set::InstanceSize(),
@@ -6449,12 +6474,16 @@ class ArraySerializationCluster : public SerializationCluster {
 class ArrayDeserializationCluster
     : public AbstractInstanceDeserializationCluster {
  public:
-  explicit ArrayDeserializationCluster(bool is_canonical, intptr_t cid)
-      : AbstractInstanceDeserializationCluster("Array", is_canonical),
+  explicit ArrayDeserializationCluster(intptr_t cid,
+                                       bool is_canonical,
+                                       bool is_root_unit)
+      : AbstractInstanceDeserializationCluster("Array",
+                                               is_canonical,
+                                               is_root_unit),
         cid_(cid) {}
   ~ArrayDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -6464,11 +6493,11 @@ class ArrayDeserializationCluster
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     const intptr_t cid = cid_;
-    const bool stamp_canonical = primary && is_canonical();
+    const bool stamp_canonical = is_root_unit_ && is_canonical();
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
       ArrayPtr array = static_cast<ArrayPtr>(d.Ref(id));
       const intptr_t length = d.ReadUnsigned();
@@ -6547,7 +6576,7 @@ class WeakArrayDeserializationCluster : public DeserializationCluster {
   WeakArrayDeserializationCluster() : DeserializationCluster("WeakArray") {}
   ~WeakArrayDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -6557,7 +6586,7 @@ class WeakArrayDeserializationCluster : public DeserializationCluster {
     stop_index_ = d->next_index();
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
@@ -6664,7 +6693,7 @@ class StringDeserializationCluster
                                            "String") {}
   ~StringDeserializationCluster() {}
 
-  void ReadAlloc(Deserializer* d) {
+  void ReadAlloc(Deserializer* d) override {
     start_index_ = d->next_index();
     const intptr_t count = d->ReadUnsigned();
     for (intptr_t i = 0; i < count; i++) {
@@ -6677,7 +6706,7 @@ class StringDeserializationCluster
     BuildCanonicalSetFromLayout(d);
   }
 
-  void ReadFill(Deserializer* d_, bool primary) {
+  void ReadFill(Deserializer* d_) override {
     Deserializer::Local d(d_);
 
     for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
@@ -6693,8 +6722,7 @@ class StringDeserializationCluster
                                instance_size - 1 * kWordSize) = 0;
       *reinterpret_cast<word*>(reinterpret_cast<uint8_t*>(str->untag()) +
                                instance_size - 2 * kWordSize) = 0;
-      Deserializer::InitializeHeader(str, cid, instance_size,
-                                     primary && is_canonical());
+      Deserializer::InitializeHeader(str, cid, instance_size, is_canonical());
 #if DART_COMPRESSED_POINTERS
       // Gap caused by less-than-a-word length_ smi sitting before data_.
       const intptr_t length_offset =
@@ -6732,7 +6760,7 @@ class StringDeserializationCluster
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs, bool primary) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     if (!table_.IsNull()) {
       auto object_store = d->isolate_group()->object_store();
       VerifyCanonicalSet(d, refs,
@@ -6903,7 +6931,7 @@ class VMDeserializationRoots : public DeserializationRoots {
  public:
   VMDeserializationRoots() : symbol_table_(WeakArray::Handle()) {}
 
-  bool AddBaseObjects(Deserializer* d) {
+  void AddBaseObjects(Deserializer* d) override {
     // These objects are always allocated by Object::InitOnce, so they are not
     // written into the snapshot.
 
@@ -6955,11 +6983,9 @@ class VMDeserializationRoots : public DeserializationRoots {
         d->AddBaseObject(StubCode::EntryAt(i).ptr());
       }
     }
-
-    return true;  // primary
   }
 
-  void ReadRoots(Deserializer* d) {
+  void ReadRoots(Deserializer* d) override {
     symbol_table_ ^= d->ReadRef();
     if (!symbol_table_.IsNull()) {
       d->isolate_group()->object_store()->set_symbol_table(symbol_table_);
@@ -6974,7 +7000,7 @@ class VMDeserializationRoots : public DeserializationRoots {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     // Move remaining bump allocation space to the freelist so it used by C++
     // allocations (e.g., FinalizeVMIsolate) before allocating new pages.
     d->heap()->old_space()->ReleaseBumpAllocation();
@@ -7142,16 +7168,15 @@ class ProgramDeserializationRoots : public DeserializationRoots {
   explicit ProgramDeserializationRoots(ObjectStore* object_store)
       : object_store_(object_store) {}
 
-  bool AddBaseObjects(Deserializer* d) {
+  void AddBaseObjects(Deserializer* d) override {
     // N.B.: Skipping index 0 because ref 0 is illegal.
     const Array& base_objects = Object::vm_isolate_snapshot_object_table();
     for (intptr_t i = kFirstReference; i < base_objects.Length(); i++) {
       d->AddBaseObject(base_objects.At(i));
     }
-    return true;  // primary
   }
 
-  void ReadRoots(Deserializer* d) {
+  void ReadRoots(Deserializer* d) override {
     // Read roots.
     ObjectPtr* from = object_store_->from();
     ObjectPtr* to = object_store_->to_snapshot(d->kind());
@@ -7171,7 +7196,7 @@ class ProgramDeserializationRoots : public DeserializationRoots {
     d->ReadDispatchTable();
   }
 
-  void PostLoad(Deserializer* d, const Array& refs) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     auto isolate_group = d->isolate_group();
     { isolate_group->class_table()->CopySizesFromClassObjects(); }
     d->heap()->old_space()->EvaluateAfterLoading();
@@ -7275,16 +7300,15 @@ class UnitDeserializationRoots : public DeserializationRoots {
  public:
   explicit UnitDeserializationRoots(const LoadingUnit& unit) : unit_(unit) {}
 
-  bool AddBaseObjects(Deserializer* d) {
+  void AddBaseObjects(Deserializer* d) override {
     const Array& base_objects =
         Array::Handle(LoadingUnit::Handle(unit_.parent()).base_objects());
     for (intptr_t i = kFirstReference; i < base_objects.Length(); i++) {
       d->AddBaseObject(base_objects.At(i));
     }
-    return false;  // primary
   }
 
-  void ReadRoots(Deserializer* d) {
+  void ReadRoots(Deserializer* d) override {
     deferred_start_index_ = d->ReadUnsigned();
     deferred_stop_index_ = deferred_start_index_ + d->ReadUnsigned();
     for (intptr_t id = deferred_start_index_; id < deferred_stop_index_; id++) {
@@ -7345,7 +7369,7 @@ class UnitDeserializationRoots : public DeserializationRoots {
     }
   }
 
-  void PostLoad(Deserializer* d, const Array& refs) {
+  void PostLoad(Deserializer* d, const Array& refs) override {
     d->EndInstructions();
     unit_.set_base_objects(refs);
   }
@@ -8932,8 +8956,8 @@ DeserializationCluster* Deserializer::ReadCluster() {
   const bool is_immutable = UntaggedObject::ImmutableBit::decode(tags);
   Zone* Z = zone_;
   if (cid >= kNumPredefinedCids || cid == kInstanceCid) {
-    return new (Z)
-        InstanceDeserializationCluster(cid, is_canonical, is_immutable);
+    return new (Z) InstanceDeserializationCluster(
+        cid, is_canonical, is_immutable, !is_non_root_unit_);
   }
   if (IsTypedDataViewClassId(cid)) {
     ASSERT(!is_canonical);
@@ -8955,13 +8979,13 @@ DeserializationCluster* Deserializer::ReadCluster() {
       case kCodeSourceMapCid:
       case kCompressedStackMapsCid:
         return new (Z)
-            RODataDeserializationCluster(is_canonical, !is_non_root_unit_, cid);
+            RODataDeserializationCluster(cid, is_canonical, !is_non_root_unit_);
       case kOneByteStringCid:
       case kTwoByteStringCid:
       case kStringCid:
         if (!is_non_root_unit_) {
-          return new (Z) RODataDeserializationCluster(is_canonical,
-                                                      !is_non_root_unit_, cid);
+          return new (Z) RODataDeserializationCluster(cid, is_canonical,
+                                                      !is_non_root_unit_);
         }
         break;
     }
@@ -9067,20 +9091,25 @@ DeserializationCluster* Deserializer::ReadCluster() {
       return new (Z)
           TypeParameterDeserializationCluster(is_canonical, !is_non_root_unit_);
     case kClosureCid:
-      return new (Z) ClosureDeserializationCluster(is_canonical);
+      return new (Z)
+          ClosureDeserializationCluster(is_canonical, !is_non_root_unit_);
     case kMintCid:
-      return new (Z) MintDeserializationCluster(is_canonical);
+      return new (Z)
+          MintDeserializationCluster(is_canonical, !is_non_root_unit_);
     case kDoubleCid:
-      return new (Z) DoubleDeserializationCluster(is_canonical);
+      return new (Z)
+          DoubleDeserializationCluster(is_canonical, !is_non_root_unit_);
     case kInt32x4Cid:
     case kFloat32x4Cid:
     case kFloat64x2Cid:
-      return new (Z) Simd128DeserializationCluster(cid, is_canonical);
+      return new (Z)
+          Simd128DeserializationCluster(cid, is_canonical, !is_non_root_unit_);
     case kGrowableObjectArrayCid:
       ASSERT(!is_canonical);
       return new (Z) GrowableObjectArrayDeserializationCluster();
     case kRecordCid:
-      return new (Z) RecordDeserializationCluster(is_canonical);
+      return new (Z)
+          RecordDeserializationCluster(is_canonical, !is_non_root_unit_);
     case kStackTraceCid:
       ASSERT(!is_canonical);
       return new (Z) StackTraceDeserializationCluster();
@@ -9094,17 +9123,20 @@ DeserializationCluster* Deserializer::ReadCluster() {
       // We do not have mutable hash maps in snapshots.
       UNREACHABLE();
     case kConstMapCid:
-      return new (Z) MapDeserializationCluster(is_canonical, kConstMapCid);
+      return new (Z) MapDeserializationCluster(kConstMapCid, is_canonical,
+                                               !is_non_root_unit_);
     case kSetCid:
       // We do not have mutable hash sets in snapshots.
       UNREACHABLE();
     case kConstSetCid:
-      return new (Z) SetDeserializationCluster(is_canonical, kConstSetCid);
+      return new (Z) SetDeserializationCluster(kConstSetCid, is_canonical,
+                                               !is_non_root_unit_);
     case kArrayCid:
-      return new (Z) ArrayDeserializationCluster(is_canonical, kArrayCid);
+      return new (Z) ArrayDeserializationCluster(kArrayCid, is_canonical,
+                                                 !is_non_root_unit_);
     case kImmutableArrayCid:
-      return new (Z)
-          ArrayDeserializationCluster(is_canonical, kImmutableArrayCid);
+      return new (Z) ArrayDeserializationCluster(
+          kImmutableArrayCid, is_canonical, !is_non_root_unit_);
     case kWeakArrayCid:
       return new (Z) WeakArrayDeserializationCluster();
     case kStringCid:
@@ -9114,8 +9146,8 @@ DeserializationCluster* Deserializer::ReadCluster() {
 #define CASE_FFI_CID(name) case kFfi##name##Cid:
       CLASS_LIST_FFI_TYPE_MARKER(CASE_FFI_CID)
 #undef CASE_FFI_CID
-      return new (Z)
-          InstanceDeserializationCluster(cid, is_canonical, is_immutable);
+      return new (Z) InstanceDeserializationCluster(
+          cid, is_canonical, is_immutable, !is_non_root_unit_);
     case kDeltaEncodedTypedDataCid:
       return new (Z) DeltaEncodedTypedDataDeserializationCluster();
     default:
@@ -9466,7 +9498,6 @@ void Deserializer::Deserialize(DeserializationRoots* roots) {
   ASSERT(instructions_table_len == 0);
 #endif  // defined(DART_PRECOMPILED_RUNTIME)
 
-  bool primary;
   {
     // The deserializer initializes objects without using the write barrier,
     // partly for speed since we know all the deserialized objects will be
@@ -9484,7 +9515,7 @@ void Deserializer::Deserialize(DeserializationRoots* roots) {
     NoSafepointScope no_safepoint;
     refs_ = refs.ptr();
 
-    primary = roots->AddBaseObjects(this);
+    roots->AddBaseObjects(this);
 
     if (num_base_objects_ != (next_ref_index_ - kFirstReference)) {
       FATAL("Snapshot expects %" Pd
@@ -9510,7 +9541,7 @@ void Deserializer::Deserialize(DeserializationRoots* roots) {
     {
       TIMELINE_DURATION(thread(), Isolate, "ReadFill");
       for (intptr_t i = 0; i < num_clusters_; i++) {
-        clusters_[i]->ReadFill(this, primary);
+        clusters_[i]->ReadFill(this);
 #if defined(DEBUG)
         int32_t section_marker = Read<int32_t>();
         ASSERT(section_marker == kSectionMarker);
@@ -9541,7 +9572,7 @@ void Deserializer::Deserialize(DeserializationRoots* roots) {
   {
     TIMELINE_DURATION(thread(), Isolate, "PostLoad");
     for (intptr_t i = 0; i < num_clusters_; i++) {
-      clusters_[i]->PostLoad(this, refs, primary);
+      clusters_[i]->PostLoad(this, refs);
     }
   }