Expression evaluation is only supported in the scope of the current frame

As far as I can tell, it's only possible to evaluate an expression in the context of the current frame. While this is the most common use case, it should be possible to evaluate in the context of specific libraries, classes, and instances.

@rmacnak-google, feel free to add more details here if I missed anything.

Start by tracing the existing expression-evaluation entry point in Flutter DevTools and identify how it receives the current frame context. Determine how library, class, and instance contexts should be selected, then add coverage for evaluation outside the current frame and verify the existing current-frame behavior remains unchanged.

索引モデルが issue の本文から書いたものです。