fix(runtime-core): in operator returning false for built-in insta…

…nce properties in `exposeProxy`

`exposeProxy` implements a fallback logic for accessing built-in instance properties but was missing the same logic for the `in` operator in the Proxy's `has()` handler.

fix vuejs#6137

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

@@ -216,7 +216,9 @@ describe('api: expose', () => {
     }
     const root = nodeOps.createElement('div')
     render(h(Parent), root)
+    expect('$el' in childRef.value).toBe(true)
     expect(childRef.value.$el.tag).toBe('div')
+    expect('$parent' in grandChildRef.value).toBe(true)
     expect(grandChildRef.value.$parent).toBe(childRef.value)
     expect(grandChildRef.value.$parent.$parent).toBe(grandChildRef.value.$root)
   })

packages/runtime-core/src/component.ts

@@ -942,6 +942,15 @@ export function getExposeProxy(instance: ComponentInternalInstance) {
           } else if (key in publicPropertiesMap) {
             return publicPropertiesMap[key](instance)
           }
+        },
+        has(target, key: string) {
+          if (key in target) {
+            return true
+          } else if (key in publicPropertiesMap) {
+            return true
+          }
+
+          return false
         }
       }))
     )