fix(runtime-core): ensure this context for $nextTick callback

fix #2282

packages/runtime-core/src/componentPublicInstance.ts

@@ -213,7 +213,7 @@ const publicPropertiesMap: PublicPropertiesMap = extend(Object.create(null), {
   $emit: i => i.emit,
   $options: i => (__FEATURE_OPTIONS_API__ ? resolveMergedOptions(i) : i.type),
   $forceUpdate: i => () => queueJob(i.update),
-  $nextTick: () => nextTick,
+  $nextTick: i => nextTick.bind(i.proxy!),
   $watch: i => (__FEATURE_OPTIONS_API__ ? instanceWatch.bind(i) : NOOP)
 } as PublicPropertiesMap)