docs: add initial Dart style guide (#1967)

Co-authored-by: Felix Angelov <felix@shorebird.dev>

style_guides/dart.md

@@ -0,0 +1,55 @@
+# Shorebird Dart Style Guide
+
+This document outlines the style guide for all Shorebird Dart code.
+
+This guide is a supplement to
+[Effective Dart](https://dart.dev/guides/language/effective-dart) and should be
+read in the same way. If there is a conflict between this guide and Effective
+Dart, this guide takes precedence.
+
+This style guide only defines rules which are not enforced via lint rules. If
+possible, prefer using lint rules to enforce style.
+
+## AVOID mixins
+
+Mixins are a powerful feature of Dart, but they make code harder to test:
+
+1. Mixins cannot be tested directly and must be tested through the classes they
+   are mixed into.
+2. Mixins are effectively unmockable dependencies of the classes they are mixed
+   into, meaning every class that uses the mixin will need to set up and mock
+   the mixin's dependencies in its own tests.
+
+Prefer creating a new class as a `scoped` dependency instead, or adding the code
+to an existing `scoped` dependency.
+
+## PREFER nested `group`s for tests
+
+These help to make the test structure clearer and easier to read, and result in
+less duplicated code for tests that rely on the same setup.
+
+```dart
+// BAD
+group('myFunction', () {
+  test('returns null when dep1.foo is true and dep2.bar is false', () {
+    when(() => dep1.foo).thenReturn(true);
+    when(() => dep2.bar).thenReturn(false);
+
+    expect(myFunction(), isNull);
+  });
+});
+
+// GOOD
+group('myFunction', () {
+  group('when dep1.foo is true and dep2.bar is false', () {
+    setUp(() {
+      when(() => dep1.foo).thenReturn(true);
+      when(() => dep2.bar).thenReturn(false);
+    });
+
+    test('returns null', () {
+      expect(myFunction(), isNull);
+    });
+  });
+});
+```