[added] SpringArgs type

Summary: Closes #134

Reviewers: featherless, O2 Material Motion, #material_motion, O3 Material JavaScript platform reviewers

Reviewed By: featherless, O2 Material Motion, #material_motion

Tags: #material_motion

Differential Revision: http://codereview.cc/D2452

Author: appsforartists

packages/streams/src/types.ts

@@ -56,3 +56,12 @@ export interface MotionElement {
   readonly scrollPosition: ScopedReadable<Point2D> & ScopedWritable<Point2D>;
   getEvent$(type: string): MotionObservable<Event>;
 }
+
+export type SpringArgs<T> = {
+  destination: ScopedReadable<T>,
+  initialValue: ScopedReadable<T>,
+  initialVelocity: ScopedReadable<T>,
+  threshold: ScopedReadable<T>,
+  friction: ScopedReadable<number>,
+  tension: ScopedReadable<number>,
+};