types(runtime-core): provide valid type for default $emit (#1498)

test-dts/defineComponent.test-d.tsx

@@ -6,7 +6,8 @@ import {
   reactive,
   createApp,
   expectError,
-  expectType
+  expectType,
+  ComponentPublicInstance
 } from './index'
 
 describe('with object props', () => {
@@ -669,4 +670,17 @@ describe('emits', () => {
       expectError(this.$emit('nope'))
     }
   })
+
+  // without emits
+  defineComponent({
+    setup(props, { emit }) {
+      emit('test', 1)
+      emit('test')
+    }
+  })
+
+  // emit should be valid when ComponentPublicInstance is used.
+  const instance = {} as ComponentPublicInstance
+  instance.$emit('test', 1)
+  instance.$emit('test')
 })