feat: Add async to body scripts (update: Use defer instead)

[atinux]

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

• Feature

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

• 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
• New/updated tests are included
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup

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

async tag let the scripts to not block the render of the page, and since it's for SSR, the scripts don't have to be loaded before the first render.

[blake-newman]

Looks good although I can't find any specification on the behaviour of multiple async script tags. Does it keep the linear order or will it execute as soon as script is parsed.

If the latter then there could be race conditions. This may be worth an investigation.

[atinux]

Based on this answer: http://stackoverflow.com/questions/8996852/load-and-execute-order-of-scripts

It may be better to use defer instead of async to keep the order, what do you think?

[blake-newman]

Good find :)

Yeah defer is the best, as it will keep predicability.

[atinux]

Commits updated 👍