Add missing Int64.toDebugString for the native class

[osa1]

The emulated class has a toDebugString but the native class didn't.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:args	2.8.0	ready to publish	args-v2.8.0
package:async	2.13.1-wip	WIP (no publish necessary)
package:characters	1.4.1	already published at pub.dev
package:collection	1.20.0-wip	WIP (no publish necessary)
package:convert	3.1.3-wip	WIP (no publish necessary)
package:crypto	3.0.7	already published at pub.dev
package:fixnum	1.2.0-wip	WIP (no publish necessary)
package:lints	6.1.0-wip	WIP (no publish necessary)
package:logging	1.3.1-wip	WIP (no publish necessary)
package:os_detect	2.0.4-wip	WIP (no publish necessary)
package:path	1.9.2-wip	WIP (no publish necessary)
package:platform	3.1.7-wip	WIP (no publish necessary)
package:typed_data	1.4.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/collection/benchmark/legacy_quicksort.dart

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

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Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
fixnum	Breaking	1.1.1	1.2.0-wip	2.0.0 Got "1.2.0-wip" expected >= "2.0.0" (breaking changes)	⚠️

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Coverage ⚠️

File	Coverage
pkgs/fixnum/lib/src/int64_native.dart	💔 95 % ⬇️ 1 %

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