Revert "[vm] Revise Dart_IntegerToHexCString to avoid dependency on Bigint"

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index ea47c97744ba30ba1cd2fe184bc1970070ee1c31..8d440bcf0364edddb1fd2cc3a917eed460d94eb1 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -2189,7 +2189,13 @@ DART_EXPORT Dart_Handle Dart_IntegerToHexCString(Dart_Handle integer,
     RETURN_TYPE_ERROR(Z, integer, Integer);
   }
   Zone* scope_zone = Api::TopScope(Thread::Current())->zone();
-  *value = int_obj.ToHexCString(scope_zone);
+  if (int_obj.IsSmi() || int_obj.IsMint()) {
+    const Bigint& bigint =
+        Bigint::Handle(Z, Bigint::NewFromInt64(int_obj.AsInt64Value()));
+    *value = bigint.ToHexCString(scope_zone);
+  } else {
+    *value = Bigint::Cast(int_obj).ToHexCString(scope_zone);
+  }
   return Api::Success();
 }