Add iterable supports for v-for

[mymyoux]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Adds support for iterable object implementing Symbol.iterator
It shouldn't have an impact for existing code because non iterable objects are not impacted.
Vue will have the same behaviour than a for...of expression for iterables. (actual interest for using iterables)
Allows us to pass an iterable directly to a template without giving internal properties or converting it to an array

I wrote 4 new tests. No existing tests needed changes.
All tests passed. I ran tests with PhantomJS (lack of Symbol support), Firefox and Chrome.

[posva]

note: git conflict with #7171

[mitar]

Related: #5893

[mitar]

So I have been using the following for a while now: https://github.com/vuejs/vue/blob/cb062c50cdd21628dcf758fe54c84d17d75cb2ca/src/core/instance/render-helpers/render-list.js

But I thought that in #5893 it is said that this will not go into core for now?

[mitar]

Why not just ret.push(render(next.value, ret.length))?

[mymyoux]

good point I can update this

[mymyoux]

But I thought that in #5893 it is said that this will not go into core for now?

I'm aware of this but I think if you choose to implement [Symbol.iterator] you choose to break old browsers compatibility. This way your iterable object has the same behaviour in Vue than in other parts of your code.
What do you think ?

[mitar]

I personally like it, I was all for #5893.