mixin support for camel_case_types

Fixes: https://github.com/dart-lang/linter/issues/2532

Change-Id: Icbc5c9ae4bd8b50c1a4471424bd9ae3d95299257
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352976
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

pkg/linter/lib/src/rules/camel_case_types.dart

@@ -60,6 +60,7 @@ class CamelCaseTypes extends LintRule {
     registry.addFunctionTypeAlias(this, visitor);
     registry.addEnumDeclaration(this, visitor);
     registry.addExtensionTypeDeclaration(this, visitor);
+    registry.addMixinDeclaration(this, visitor);
   }
 }
 
@@ -107,4 +108,9 @@ class _Visitor extends SimpleAstVisitor<void> {
   void visitGenericTypeAlias(GenericTypeAlias node) {
     check(node.name);
   }
+
+  @override
+  void visitMixinDeclaration(MixinDeclaration node) {
+    check(node.name);
+  }
 }

pkg/linter/test/rules/camel_case_types_test.dart

@@ -76,4 +76,12 @@ macro class a { }
       lint(12, 1),
     ]);
   }
+
+  test_mixin_lowerCase() async {
+    await assertDiagnostics(r'''
+mixin m { }
+''', [
+      lint(6, 1),
+    ]);
+  }
 }