fix(core): fix type check for Date prop

Now it checks a prop by `[object Date]` instead of `instanceof Date`

Author: kikuchan

src/core/util/props.js

@@ -8,6 +8,7 @@ import {
   toRawType,
   hyphenate,
   capitalize,
+  isDate,
   isPlainObject
 } from 'shared/util'
 
@@ -166,6 +167,8 @@ function assertType (value: any, type: Function, vm: ?Component): {
     valid = isPlainObject(value)
   } else if (expectedType === 'Array') {
     valid = Array.isArray(value)
+  } else if (expectedType === 'Date') {
+    valid = isDate(value)
   } else {
     try {
       valid = value instanceof type

src/shared/util.js

@@ -63,6 +63,10 @@ export function isRegExp (v: any): boolean {
   return _toString.call(v) === '[object RegExp]'
 }
 
+export function isDate (v: any): boolean {
+  return _toString.call(v) === '[object Date]'
+}
+
 /**
  * Check if val is a valid array index.
  */