[added] PropertyObservable

Summary:
`ReactiveProperty` is a class, but since JavaScript is duck-typed, we ought to only care that the object conforms to the right shape.  Thus, `PropertyObservable` - the shape that `ReactiveProperty` implements.

I did some experimenting.  You can treat a class as an interface, but the implementor must have an identical shape, [even private members
](http://www.typescriptlang.org/play/index.html#src=class%20Hi%20%7B%0A%20%20private%20_other%20%3D%20true%3B%0A%0A%20%20thing%28%29%3A%20boolean%20%7B%0A%20%20%20%20return%20false%0A%20%20%7D%0A%7D%0A%0A%2F%2F%20%2F%2F%20This%20doesn't%20seem%20to%20matter%0A%2F%2F%20interface%20Hi%20%7B%0A%2F%2F%20%20%20thing%28%29%3A%20boolean%3B%0A%2F%2F%20%7D%0A%0Aclass%20Bye%20implements%20Hi%20%7B%0A%20%20thing%20%3D%20%28%29%20%3D%3E%20true%0A%7D%0A%0Afunction%20doThing%28to%3A%20Hi%29%20%7B%0A%20%20return%20to.thing%28%29%0A%7D%0A%0AdoThing%28new%20Bye%28%29%29%3B).

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

Reviewed By: featherless, O2 Material Motion, #material_motion

Subscribers: featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2500

packages/experimental-addons/src/TogglableProperty.ts

@@ -20,12 +20,11 @@ import {
 } from 'indefinite-observable';
 
 import {
+  PropertyObservable,
   ReactiveProperty,
-  ScopedReadable,
-  ScopedWritable,
 } from 'material-motion-streams';
 
-export class TogglableProperty<T> implements Observable<T>, ScopedReadable<T>, ScopedWritable<T> {
+export class TogglableProperty<T> implements PropertyObservable<T> {
   _property = new ReactiveProperty<T>();
   _isOn = false;
   _onValue: T;

packages/streams/src/properties/ReactiveProperty.ts

@@ -25,11 +25,12 @@ import {
 } from '../observables/IndefiniteSubject';
 
 import {
+  PropertyObservable,
   ScopedReadable,
   ScopedWritable,
 } from '../types';
 
-export class ReactiveProperty<T> implements Observable<T>, ScopedReadable<T>, ScopedWritable<T> {
+export class ReactiveProperty<T> implements PropertyObservable<T> {
   // ReactiveProperty delegates all of its reactive functionality to an internal
   // instance of IndefiniteSubject.
   //

packages/streams/src/types.ts

@@ -51,16 +51,17 @@ export interface ScopedWritable<T> {
   write: Write<T>;
 }
 
+export interface PropertyObservable<T> extends Observable<T>, ScopedReadable<T>, ScopedWritable<T> {}
 
 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>,
+  destination: PropertyObservable<T>,
+  initialValue: PropertyObservable<T>,
+  initialVelocity: PropertyObservable<T>,
   threshold: ScopedReadable<number>,
   friction: ScopedReadable<number>,
   tension: ScopedReadable<number>,