Save list of sprite physics joints

packages/flutter_sprites/lib/physics_joint.dart

@@ -25,6 +25,7 @@ abstract class PhysicsJoint {
     if (_joint == null) {
       _physicsNode = physicsNode;
       _joint = _createB2Joint(physicsNode);
+      _physicsNode._joints.add(this);
     }
   }
 
@@ -32,6 +33,7 @@ abstract class PhysicsJoint {
     if (_joint != null && _active) {
       _physicsNode.b2World.destroyJoint(_joint);
       _joint = null;
+      _physicsNode._joints.remove(this);
     }
     _active = false;
   }

packages/flutter_sprites/lib/physics_node.dart

@@ -31,6 +31,8 @@ class PhysicsNode extends Node {
 
   _ContactHandler _contactHandler;
 
+  List<PhysicsJoint> _joints = [];
+
   List<box2d.Body> _bodiesScheduledForDestruction = [];
 
   double b2WorldToNodeConversionFactor = 10.0;