[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 0b865fdf3952144666315a7d7d2f5245cd8e8f63..693611af854c63ffa05b7832d731a59d65c3e15e 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -2190,13 +2190,7 @@ DART_EXPORT Dart_Handle Dart_IntegerToHexCString(Dart_Handle integer,
     RETURN_TYPE_ERROR(Z, integer, Integer);
   }
   Zone* scope_zone = Api::TopScope(Thread::Current())->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);
-  }
+  *value = int_obj.ToHexCString(scope_zone);
   return Api::Success();
 }