Provided values in mixin are not accessible

[Tidyzq]

Version

2.5.21

Reproduction link

https://codepen.io/anon/pen/oJLdMN

Open the link and see the error message in dev console.

Steps to reproduce

Create two mixins with a provide object using Symbol as it's key field. Then assign these mixins to a component.

What is expected?

The child component can access two provide object without any error.

What is actually happening?

The child component can only access the last provide object, while the first one is undefined.

---

Diving into the source code, the reason of this issue is that Vue uses Object.keys to iterate over the provide object while merging mixins. However, Object.keys won't return Symbol keys. see the code.
Maybe we should use for ... in loop, which will iterate through Symbol keys correctly.

[posva]

It doesn't seem to be related to Symbols but rather using provide in a mixin as this also fails:

var provideMixin1 = {
  provide: {
    foo: 'it is foo'
  }
}

var Parent = {
  mixin: [
    provideMixin1,
  ],
  template: '<div><slot /></div>'
}

var Child = {
  inject: ['foo'],
  template: '<div>foo: {{ foo }} bar:</div>',
}

new Vue({
  components: {
    Parent,
    Child,
  },
  template: '<parent><child /></parent>'
}).$mount('#app')

[Tidyzq]

I'm sorry that I got a typo...
It should be mixins rather than mixin, but the Symboled keys still can't be treated correctly.
https://codepen.io/anon/pen/NeroQJ

[yyx990803]

This is addressed by #7926 (pending merge for next minor)

[posva]

Ups, my bad, sorry!