refactor: cache index for callbacks, remove redundant watcher

src/core/observer/watcher.js

@@ -181,11 +181,10 @@ export default class Watcher {
       const value = this.get()
       if (
         value !== this.value ||
-        // Deep watchers and watchers on Object/Arrays should fire even
+        // Watchers on Object/Arrays should fire even
         // when the value is the same, because the value may
         // have mutated.
-        isObject(value) ||
-        this.deep
+        isObject(value)
       ) {
         // set new value
         const oldValue = this.value

src/core/util/next-tick.js

@@ -12,10 +12,10 @@ let pending = false
 
 function flushCallbacks () {
   pending = false
-  const copies = callbacks.slice(0)
-  callbacks.length = 0
-  for (let i = 0; i < copies.length; i++) {
-    copies[i]()
+  const currentLength = callbacks.length
+  const currentCallback = callbacks.splice(0)
+  for (let i = 0; i < currentLength; i++) {
+    currentCallback[i]()
   }
 }