fix #5592: comment vnode should not be merged into text vnode.

[maggiehe]

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:

[blake-newman]

This fixes one issue with vnode issues. svg is not correctly generating vnode which causes further issues with SSR. I will further investigate this, there will be some changes i will recommend but will do once i have discovered the further issues.

[blake-newman]

Upon further investigation i have narrowed my issue down to my SVG component being an asynchronous component.

In the docs:

This makes it a bit tricky to use async components at arbitrary locations in your app (we will likely improve this in the future). However, it works seamlessly if you do it at the route level.

I think this PR is fine other than probably needing an isFalse helper.

[maggiehe]

@blake-newman Agree, I have added it beside the isTrue method.

[blake-newman]

I'll ping @yyx990803 to see if he can review to ensure that everything is covered and there are no other possible side affects.

is this crucial for this to be fixed ASAP for you?

[maggiehe]

It is not very urgent, I have been temporarily downgraded the version to 2.2.6.
Thanks for your help:)