Add a `master` CI

[domesticmouse]

This change is required to add CI coverage for https://github.com/flutter/samples/pull/1378

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.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] All existing and new tests are passing.

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