Configure Gemini code review

.gemini/config.yaml

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+# See https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#custom-configurations
+
+have_fun: false # We are serious people in the Dart team!
+code_review:
+  disable: false
+  comment_severity_threshold: MEDIUM
+  max_review_comments: 100
+  pull_request_opened:
+    help: true
+    summary: true
+    code_review: true
+ignore_patterns: []

.gemini/styleguide.md

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+# Dart Style Guide
+
+This style guide outlines the coding conventions for contributions to the
+`dart-lang/**` repositories.
+
+## Best Practices
+
+- Follow [the contribution guide](CONTRIBUTING.md).
+- Code should be tested.
+- Write code follow the guidance described in [effective Dart](https://dart.dev/effective-dart).
+
+## General Philosophy
+
+- Optimize for readability - code is read more often than it is written.
+
+## Documentation
+
+- At least all public members should have documentation, answering the *why*.
+- Assume the reader does not know everything. Link to definitions.
+- Provide sample code.