With the Marshal Plan, when create an instance of a manager, we need to talk to the system manager. Unfortunately sometimes these manager instances can be created before the Application class is created and/or before the systemManager property is set on the application. So to get the reference to the system manager from the manager instances, we can't use ApplicationGlobals.application.systemManager. We need to use SystemManagerGlobals.topLevelSystemManagers instead. This property is set in the SystemManager constructor, and fortunately for us, Flash runs your root display object's constructor before anything else, even before static initializers (atleast ones not defined in SystemManager or a dependency of it).
Bugs: SDK-16207, SDK-16208, SDK-16209