Three JS dashed line material not showing

Geometry.computeLineDistances() has been deprecated, so Derte Trdelnik's answer can not work now.

Use Line.computeLineDistances() instead.

geometry.computeLineDistances();

http://threejs.org/docs/#api/core/Geometry

.lineDistances

An array containing distances between vertices for Line geometries. This is required for LinePieces/LineDashedMaterial to render correctly. Line distances can also be generated with computeLineDistances.