[analyzer/ffi] Fix spurious FFI errors

Closes: https://github.com/dart-lang/sdk/issues/52298
Change-Id: Ibfccd34a877d5786f4720790b5ff88dcc643c9d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/301980
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

pkg/analyzer/lib/src/generated/ffi_verifier.dart

@@ -156,6 +156,7 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
       }
     }
     super.visitClassDeclaration(node);
+    inCompound = false;
   }
 
   @override

tests/ffi/regress_52298_test.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+final class Foo extends Struct {
+  external Pointer<Void> field;
+}
+
+mixin Bar {
+  late final field = 123;
+}
+
+void main() {
+  print('something');
+}