Updated fragment shader sample to use uniform-by-name accessor

[gaaclarke]

Do not land until the following are on stable:

• https://github.com/flutter/flutter/pull/176728
• https://github.com/flutter/flutter/pull/176980

Pre-launch Checklist

• [x] I read the Flutter Style Guide recently, and have followed its advice.
• [x] I signed the CLA.
• [x] I read the Contributors Guide.
• [ ] I have added sample code updates to the changelog.
• [x] I updated/added relevant documentation (doc comments with ///).

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