Object pool with support for untagged entries.

runtime/vm/assembler_arm64.h

Index: runtime/vm/assembler_arm64.h
diff --git a/runtime/vm/assembler_arm64.h b/runtime/vm/assembler_arm64.h
index 98e3f36b92d905e2d07c2b91b338d01e8a0df4bb..70d37b32baded89551c4f00d50139b3bf0405682 100644
--- a/runtime/vm/assembler_arm64.h
+++ b/runtime/vm/assembler_arm64.h
@@ -476,11 +476,11 @@ class Assembler : public ValueObject {
     return buffer_.pointer_offsets();
   }
 
-  const GrowableObjectArray& object_pool_data() const {
-    return object_pool_.data();
-  }
+  ObjectPoolWrapper& object_pool_wrapper() { return object_pool_wrapper_; }
 
-  ObjectPool& object_pool() { return object_pool_; }
+  RawObjectPool* MakeObjectPool() {
+    return object_pool_wrapper_.MakeObjectPool();
+  }
 
   bool use_far_branches() const {
     return FLAG_use_far_branches || use_far_branches_;
@@ -1418,8 +1418,7 @@ class Assembler : public ValueObject {
  private:
   AssemblerBuffer buffer_;  // Contains position independent code.
 
-  // Objects and patchable jump targets.
-  ObjectPool object_pool_;
+  ObjectPoolWrapper object_pool_wrapper_;
 
   int32_t prologue_offset_;