This looks like the inline debugging VS Code offers. You can toggle it off through Settings > Debug: Inline Values > Off.
Additionally, there are two settings to the Dart extension which may control this behavior:
dart.evaluateGettersInDebugViews
Default: true.
Whether to evaluate getters in order to display them in debug views (such as the Variables, Watch and Hovers views).
dart.showGettersInDebugViews
Default: true.
Whether to show getters in order to display them in debug views (such as the Variables, Watch and Hovers views). If evaluateGettersInDebugViews is true getters will be eagerly evaluated, otherwise they will require clicking to evaluate.