Scene can be retrieved from the object

Scene is optional, as most objects will have the getScene function that
will deliver the scene.

src/Physics/babylon.physicsImpostor.ts

@@ -18,6 +18,7 @@ module BABYLON {
         getChildMeshes?(): Array<AbstractMesh>;
         getVerticesData?(kind: string): Array<number> | Float32Array;
         getIndices?(): Array<number>| Int32Array;
+        getScene?() : Scene;
     }
 
     export class PhysicsImpostor {
@@ -48,8 +49,13 @@ module BABYLON {
             otherImpostor: PhysicsImpostor
         }>;
 
-        constructor(public object: IPhysicsEnabledObject, public type: number, private _options: PhysicsImpostorParameters = { mass: 0 }, private _scene: Scene) {
-            this._physicsEngine = this._scene.getPhysicsEngine();
+        constructor(public object: IPhysicsEnabledObject, public type: number, private _options: PhysicsImpostorParameters = { mass: 0 }, scene?: Scene) {
+            //legacy support for old syntax.
+            if(!scene && object.getScene) {
+                scene = object.getScene()
+            }
+            
+            this._physicsEngine = scene.getPhysicsEngine();
             if (!this._physicsEngine) {
                 Tools.Error("Physics not enabled. Please use scene.enablePhysics(...) before creating impostors.")
             } else {