OptionalcontentThe child widget to wrap.
OptionalnodeThe optional node to use for the widget.
If a node is provided, the widget will assume full ownership and control of the node, as if it had created the node itself.
The default is a new <div>.
OptionalrevealAn optional promise for when the content is ready to be revealed.
OptionaltagThe optional element tag, used for constructing the widget's node.
If a pre-constructed node is provided via the node arg, this
value is ignored.
OptionaltoolbarThe toolbar to use for the widget. Defaults to an empty toolbar.
An options object for main area widget subclasses providing their own default content.
Notes
This makes it easier to have a subclass that provides its own default content. This can go away once we upgrade to TypeScript 2.8 and have an easy way to make a single property optional, ala https://stackoverflow.com/a/46941824