Support untagged object pool entries.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index a257d770bcfab6cd7a53b0fd433ac64a0b039fa3..1cbdc595c13788c9afaf41bcc8628fef4ef65207 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -762,7 +762,7 @@ class Assembler : public ValueObject {
   void Jmp(const ExternalLabel* label, Register pp);
   void J(Condition condition, const ExternalLabel* label, Register pp);
   void CallPatchable(const ExternalLabel* label);
-  void Call(const ExternalLabel* label, Register pp);
+  void Call(const ExternalLabel* label, Register pp, bool emit_fixup = true);
   // Unaware of write barrier (use StoreInto* methods for storing to objects).
   // TODO(koda): Add StackAddress/HeapAddress types to prevent misuse.
   void StoreObject(const Address& dst, const Object& obj, Register pp);
@@ -897,11 +897,8 @@ class Assembler : public ValueObject {
   const ZoneGrowableArray<intptr_t>& GetPointerOffsets() const {
     return buffer_.pointer_offsets();
   }
-  const GrowableObjectArray& object_pool_data() const {
-    return object_pool_.data();
-  }
 
-  ObjectPool& object_pool() { return object_pool_; }
+  ObjectPoolHelper& object_pool() { return object_pool_; }
 
   void FinalizeInstructions(const MemoryRegion& region) {
     buffer_.FinalizeInstructions(region);
@@ -1025,11 +1022,15 @@ class Assembler : public ValueObject {
   static bool IsSafe(const Object& object) { return true; }
   static bool IsSafeSmi(const Object& object) { return object.IsSmi(); }
 
+  static const int kNumFixedObjectEntries = 3;
+  static const int kNumFixedEntries = 5;
+  COMPILE_ASSERT(kNumFixedObjectEntries <= kNumFixedEntries);
+
  private:
   AssemblerBuffer buffer_;
 
   // Objects and jump targets.
-  ObjectPool object_pool_;
+  ObjectPoolHelper object_pool_;
 
   intptr_t prologue_offset_;