support no fallback on browsers which do not support history API, fix #1456

[JounQin]

No description provided.

[posva]

Could you add a test for it in the api.spec.js file please?

[JounQin]

@posva Sorry but it should only work on IE9, and it seems there is no spec for fallback to hash mode before, so actually I have trouble about how to write the test spec.

[posva]

I meant that you can test that the mode is actually forced to 'history' even when there's no browser support for pushState (https://github.com/vuejs/vue-router/blob/dev/src/util/push-state.js#L6)

[JounQin]

But it will always fallback to abstract on server/Node.js and it should not be broken.

https://github.com/vuejs/vue-router/blob/dev/src/index.js#L49

Then I pretend it as browser inside, I don't know does it meet your requirement.

[posva]

Sorry if I was too pushy there, I meant to have an open discussion about it 😅
Yes, it should default to abstract in Node. What you wrote looks pretty much like what I was thinking (without the dist files ofc, if you can remove that from the commit, that would be great 😄 )

[JounQin]

But the spec indeed requires the dist files, because as I mentioned in comment: "we do not use Router top above directly because inBrowser has been initialized inside".

[yyx990803]

The dist file will be overwritten between builds... this won't work. Let's just remove the spec.

[JounQin]

@yyx990803 I've reset the last commit.