[analysis_server] Fix completion error when a typedef points to a mixin

This fixes an exception that prevents completion from working if a typedef points at a mixin, which is something Flutter has:

```dart
typedef RenderConstrainedLayoutBuilder<LayoutInfoType, ChildType extends RenderObject> =
    RenderAbstractLayoutBuilderMixin<LayoutInfoType, ChildType>;

mixin RenderAbstractLayoutBuilderMixin<LayoutInfoType, ChildType extends RenderObject>
    on RenderObjectWithChildMixin<ChildType>, RenderObjectWithLayoutCallbackMixin
```

Fixes #61197

Change-Id: Ibf31612d34893f2183a7da513359bdbff6a5c066
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/441962
Reviewed-by: Keerti Parthasarathy <[email protected]>
Commit-Queue: Keerti Parthasarathy <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/services/completion/dart/declaration_helper.dart

@@ -1832,6 +1832,7 @@ class DeclarationHelper {
       return;
     }
     if (checkVisibilty &&
+        constructors.isNotEmpty &&
         !visibilityTracker.isVisible(
           element: constructors.first.enclosingElement,
           importData: importData,

pkg/analysis_server/test/src/services/completion/dart/completion_test.dart

@@ -100,6 +100,19 @@ abstract class C {
 ''');
     assertHasNoCompletion('C.c');
   }
+
+  /// Ensures completion does not error when a typedef points at a mixin because
+  /// there are no constructors.
+  Future<void> test_typeDef_toMixin_noError_issue61197() async {
+    await getTestCodeSuggestions('''
+typedef T = M;
+mixin M {}
+
+void main() {
+  ^
+}
+''');
+  }
 }
 
 @reflectiveTest