fix(watch): flush:pre watchers should not fire if state change causes

owner component to unmount

fix #2291

packages/runtime-core/__tests__/apiWatch.spec.ts

@@ -509,7 +509,8 @@ describe('api: watch', () => {
     expect(cb).not.toHaveBeenCalled()
   })
 
-  it('should fire on component unmount w/ flush: pre', async () => {
+  // #2291
+  it('should not fire on component unmount w/ flush: pre', async () => {
     const toggle = ref(true)
     const cb = jest.fn()
     const Comp = {
@@ -527,7 +528,7 @@ describe('api: watch', () => {
     expect(cb).not.toHaveBeenCalled()
     toggle.value = false
     await nextTick()
-    expect(cb).toHaveBeenCalledTimes(1)
+    expect(cb).not.toHaveBeenCalled()
   })
 
   // #1763