do not stringify watcher function upfront

Author: yyx990803

src/watcher.js

@@ -40,7 +40,7 @@ export default function Watcher (vm, expOrFn, cb, options) {
   var isFn = typeof expOrFn === 'function'
   this.vm = vm
   vm._watchers.push(this)
-  this.expression = isFn ? expOrFn.toString() : expOrFn
+  this.expression = expOrFn
   this.cb = cb
   this.id = ++uid // uid for batching
   this.active = true