Revert "Completely new cross-platform API introduced"

[natebosch]

This reverts commit 0def20c.

Temporarily reverting to fix long-tail issues with internal usage
patterns.

[github-actions]

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// Copyright (c) 2026, 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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Unused Dependencies ✔️

Package	Status
platform	✔️ All dependencies utilized correctly.

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Coverage ⚠️

File	Coverage
pkgs/platform/example/bin/example.dart	💔 Not covered
pkgs/platform/lib/platform.dart	💔 Not covered
pkgs/platform/lib/src/interface/local_platform.dart	💚 100 %
pkgs/platform/lib/src/interface/platform.dart	💚 75 %
pkgs/platform/lib/src/testing/fake_platform.dart	💚 99 %

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
platform	None	3.1.6	3.1.7-wip	3.1.7-wip	✔️

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API leaks ✔️

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Package	Leaked API symbol	Leaking sources

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