Highlight a Flutter widget in Studio mode

[vusters]

In Studio mode, we need the ability to highlight the Flutter widget when that widget is selected in the Tree. This is especially important in Visual mode (add propertyPanelEnabled=true to the URL).

• The highlight should ONLY exists in Studio mode (it should be a no op without --dart-define=studio=true and has no performance penalty in Production)
• The highlight (thichness of 2) should NOT make the widget larger. Instead it should overlay itself on top of the widget, adding no additional pixels.
• From React perhaps we pass an xpath to FLutter, which will look for the widget and highlight it?