TreeItem 
  Parameters
label
A human-readable string describing this item
collapsibleState 
TreeItemCollapsibleState of the tree item. Default is TreeItemCollapsibleState.None
constructor(label: TreeItemLabel, collapsibleState: TreeItemCollapsibleState = definedExternally)(source)
constructor(resourceUri: Uri, collapsibleState: TreeItemCollapsibleState = definedExternally)(source)
Parameters
resourceUri 
The Uri of the resource representing this item.
collapsibleState 
TreeItemCollapsibleState of the tree item. Default is TreeItemCollapsibleState.None