Small performance improvement in watcher.js (#4069)

* Minor minification in codegen/index.js

* Small performance improvement in watcher.js

* Use a faster return pattern in watcher.js (@HerringtonDarkholme suggestion)

Author: sapics

src/compiler/codegen/index.js

@@ -221,9 +221,9 @@ function genText (text: ASTText | ASTExpression): string {
 function genSlot (el: ASTElement): string {
   const slotName = el.slotName || '"default"'
   const children = genChildren(el)
-  return children
-    ? `_t(${slotName},${children})`
-    : `_t(${slotName})`
+  return `_t(${slotName}${
+    children ? `,${children}` : ''
+  })`
 }
 
 // componentName is el.component, take it as argument to shun flow's pessimistic refinement

src/core/observer/watcher.js

@@ -230,30 +230,30 @@ export default class Watcher {
  * is collected as a "deep" dependency.
  */
 const seenObjects = new Set()
-function traverse (val: any, seen?: Set) {
+function traverse (val: any) {
+  seenObjects.clear()
+  _traverse(val, seenObjects)
+}
+
+function _traverse (val: any, seen: Set) {
   let i, keys
-  if (!seen) {
-    seen = seenObjects
-    seen.clear()
-  }
   const isA = Array.isArray(val)
-  const isO = isObject(val)
-  if ((isA || isO) && Object.isExtensible(val)) {
-    if (val.__ob__) {
-      const depId = val.__ob__.dep.id
-      if (seen.has(depId)) {
-        return
-      } else {
-        seen.add(depId)
-      }
-    }
-    if (isA) {
-      i = val.length
-      while (i--) traverse(val[i], seen)
-    } else if (isO) {
-      keys = Object.keys(val)
-      i = keys.length
-      while (i--) traverse(val[keys[i]], seen)
+  if ((!isA && !isObject(val)) || !Object.isExtensible(val)) {
+    return
+  }
+  if (val.__ob__) {
+    const depId = val.__ob__.dep.id
+    if (seen.has(depId)) {
+      return
     }
+    seen.add(depId)
+  }
+  if (isA) {
+    i = val.length
+    while (i--) _traverse(val[i], seen)
+  } else {
+    keys = Object.keys(val)
+    i = keys.length
+    while (i--) _traverse(val[keys[i]], seen)
   }
 }