Description

Adds a package with a common base class for platform interface.

The PlatformInterface class provides common functionality for enforcing that platform implementations extend the platform interface with extends rather than implement it with implements.

This is based on the existing code from the url_laucher_platform_interface, and the improvements proposed in #2326

Related Issues

flutter/flutter#43368

Checklist

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
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• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.