code: function( bodies, dt ){
  
              var world = this._world;
  
              this.integrateVelocities( bodies, dt );
  
              if ( world ){
                  world.emit('integrate:velocities', {
                      bodies: bodies,
                      dt: dt
                  });
              }
  
              this.integratePositions( bodies, dt );
  
              if ( world ){
                  world.emit('integrate:positions', {
                      bodies: bodies,
                      dt: dt
                  });
              }
  
              return this;
          }
docs: Integrator#integrate( bodies, dt ) -> this
  - bodies (Array): List of bodies to integrate
  - dt (Number): Timestep size
  
  Integrate bodies by timestep.
  
  Will emit `integrate:velocities` and `integrate:positions`
  events on the world.