refactor(runtime-core): remove emit return value

BREAKING CHANGE: this.$emit() and setupContext.emit() no longer
return values. For logic that relies on return value of listeners,
the listener should be declared as an `onXXX` prop and be called
directly. This still allows the parent component to pass in
a handler using `v-on`, since `v-on:foo` internally compiles
to `onFoo`.

    ref: https://github.com/vuejs/rfcs/pull/16

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

@@ -0,0 +1,47 @@
+import { ref, render, h, nodeOps, nextTick } from '@vue/runtime-test'
+
+describe('component: slots', () => {
+  // TODO more tests for slots normalization etc.
+
+  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 stable
+    // -> 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)
+  })
+})