Use prototype toString to prevent bad serialization (vuejs#569)

* Edge case of incorrectly serialized function 

Should fix vuejs#568

* Function.toString hijacking example

* Use prototype toString

shells/dev/target/NativeTypes.vue

@@ -33,6 +33,17 @@
 import { mapState, mapGetters, mapMutations } from 'vuex'
 import CompDef from './Other.vue'
 
+function setToString (func, string) {
+  return Object.defineProperty(func, 'toString', {
+    configurable: true,
+    enumerable: false,
+    value: () => string,
+    writable: true
+  })
+}
+
+const aWeirdFunction = setToString(function weird (a, b, c) {}, 'foo')
+
 export default {
   components: {
     TestComponent: {
@@ -53,6 +64,7 @@ export default {
       hello: function foo (a, b, c) {},
       hey: function empty () {},
       anon: function (foo, bar) {},
+      aWeirdFunction,
       arrow: (a, b) => {},
       def: CompDef,
       def2: {

src/util.js

@@ -117,9 +117,9 @@ function replacer (key) {
       return encodeCache.cache(val, () => getCustomSetDetails(val))
     } else if (val instanceof RegExp) {
       // special handling of native type
-      return `[native RegExp ${val.toString()}]`
+      return `[native RegExp ${RegExp.prototype.toString.call(val)}]`
     } else if (val instanceof Date) {
-      return `[native Date ${val.toString()}]`
+      return `[native Date ${Date.prototype.toString.call(val)}]`
     } else if (val.state && val._vm) {
       return encodeCache.cache(val, () => getCustomStoreDetails(val))
     } else if (val.constructor && val.constructor.name === 'VueRouter') {
@@ -230,7 +230,9 @@ export function getCustomComponentDefinitionDetails (def) {
 }
 
 export function getCustomFunctionDetails (func) {
-  const args = func.toString().match(/\(.*\)/)[0]
+  const string = Function.prototype.toString.call(func) || ''
+  const matches = string.match(/\(.*\)/)
+  const args = matches ? matches[0] : '(?)'
   return {
     _custom: {
       type: 'function',