In 4.3 the widget controls were fully initialized up front along with the sidebar controls. The sidebar control depended (unnecessarily) on the widget control to ensure that `wp.customize.Widgets.savedWidgetIds` was defined. So after [34563] there could be a situation where the widgets are added/removed from a sidebar before their controls are initialized (if the sidebar section is never expanded), resulting in an error attempting to get a property off of an undefined value. So this change does the right thing and defines `savedWidgetIds` up front.
Also changes the `savedWidgetIds` variable type from an array to an object, to match how it is used as a dictionary lookup.
See #33901.
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git-svn-id: http://core.svn.wordpress.org/trunk@34848 1a063a9b-81f0-0310-95a4-ce76da25c4cd