feat(core): respect $stable slots flag per RFC

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

@@ -1,3 +1,5 @@
+import { h, ref, render, nodeOps, nextTick } from '@vue/runtime-test'
+
 describe('renderer: component', () => {
   test.todo('should work')
 
@@ -7,5 +9,47 @@ describe('renderer: component', () => {
 
   test.todo('componentProps')
 
-  test.todo('componentSlots')
+  describe('slots', () => {
+    test('should respect $stable flag', async () => {
+      const flag1 = ref(1)
+      const flag2 = ref(2)
+      const spy = jest.fn()
+
+      const Child = () => {
+        spy()
+        return 'child'
+      }
+
+      const App = {
+        setup() {
+          return () => [
+            flag1.value,
+            h(
+              Child,
+              { n: flag2.value },
+              {
+                foo: () => 'foo',
+                $stable: true
+              }
+            )
+          ]
+        }
+      }
+
+      render(h(App), nodeOps.createElement('div'))
+      expect(spy).toHaveBeenCalledTimes(1)
+
+      // parent re-render, props didn't change, slots are stasble
+      // -> child should not update
+      flag1.value++
+      await nextTick()
+      expect(spy).toHaveBeenCalledTimes(1)
+
+      // parent re-render, props changed
+      // -> child should update
+      flag2.value++
+      await nextTick()
+      expect(spy).toHaveBeenCalledTimes(2)
+    })
+  })
 })