message: fix: check enterToClass/leaveToClass existence before adding it
  related:
diff: @@ -69,9 +69,12 @@ export function nextFrame (fn: Function) {
  }
  export function addTransitionClass (el: any, cls: string) {
  - (el._transitionClasses || (el._transitionClasses = [])).push(cls)
  + const transitionClasses = el._transitionClasses || (el._transitionClasses = [])
  + if (transitionClasses.indexOf(cls) < 0) {
  + transitionClasses.push(cls)
  addClass(el, cls)
  }
  +}
  export function removeTransitionClass (el: any, cls: string) {
  if (el._transitionClasses) {
  