I've found the FeatureLayer API reference here.
One of the FeatureLayer properties is labelingInfo of type LabelClass. The labelingInfo details says that "There is no support for labeling in 2D. Labeling is only supported in 3D SceneViews." I want to add labels to a 2D map.
I've also found the TextSymbol Class, which says it is "used to define the graphic for displaying labels on a FeatureLayer." If that's the case, I can't figure out what FeatureLayer property to include a TextSymbol.