fix watcher path check

Author: yyx990803

src/core/observer/watcher.js

@@ -66,7 +66,7 @@ export default class Watcher {
       if (!this.getter) {
         this.getter = function () {}
         process.env.NODE_ENV !== 'production' && warn(
-          'Failed watching path: ' + expOrFn +
+          `Failed watching path: "${expOrFn}" ` +
           'Watcher only accepts simple dot-delimited paths. ' +
           'For full control, use a function instead.',
           vm

src/core/util/lang.js

@@ -23,7 +23,7 @@ export function def (obj: Object, key: string, val: any, enumerable?: boolean) {
 /**
  * Parse simple path.
  */
-const bailRE = /[^\w\.]/
+const bailRE = /[^\w\.\$]/
 export function parsePath (path: string): any {
   if (bailRE.test(path)) {
     return