[material_ui] Add the Material shapes.

[ksokolovskyi]

Description

Adds the Material shapes library to material_ui, a Dart port of the AndroidX graphics-shapes library and the Compose MaterialShapes catalog. These APIs are the foundation for upcoming M3 Expressive components (loading indicator, wavy progress indicators, etc.) and let app developers use the Material shape catalog and shape-morphing animations directly.

New public API:

• RoundedPolygon, CornerRounding, CubicBezier, Feature: creation and transformation of rounded polygonal shapes.
• Morph: smooth morphing between two RoundedPolygons.
• MaterialShapes: the catalog of 35 predefined Material Design shapes (pictured below).
• MaterialShapeBorder: an OutlinedBorder that draws a RoundedPolygon, so the shapes plug into the existing ShapeDecoration/shape APIs and animate through the usual border lerping.

The port keeps the geometry and behavior of the Kotlin original (unit tests are ported from AndroidX alongside new ones), while adapting the API surface to Dart/Flutter conventions. Every shape in MaterialShapes.all is covered by a golden test. Also includes a new example that morphs through the whole catalog.

Closes flutter/flutter#184934

Shapes catalog

Example

material_shapes_example.mov

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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[gemini-code-assist]

Code Review

This pull request introduces a shapes library for creating and morphing rounded polygonal shapes, including classes such as RoundedPolygon, CornerRounding, CubicBezier, Feature, and Morph, along with predefined Material Design shapes in MaterialShapes and a MaterialShapeBorder class. Feedback on these changes highlights several issues: an invalid enum syntax in the example painter, potential division-by-zero or NaN bugs in LengthMeasurer and RoundedPolygon.normalized, a typo and incorrect parameter name in an error message, and performance optimization opportunities in Morph.toCubics and CubicBezier operators to avoid list allocations.