Toggle Constrain Proportions.sketchplugin

// Toggles the currently selected item(s) 'Constrain Proportions' setting
for (var i=0; i < [selection count]; i++) {
  var s = [selection objectAtIndex:i]
  [[s frame] setConstrainProportions:![s constrainProportions]]
}
// This is needed to update the inspector's icon
[doc reloadInspector]