Implemented async attributes and content.

[radekmie]

This is an extension of #424, where both async attributes and content are now supported. That means the following code is now correct:

Template.example.helpers({
  getClass: async () => 'dynamic',
  getContent: async () => 'Hello there!',
});

<template name="example">
  <span class="static {{getClass}}">{{getContent}}</span>
</template>

Just like the #if and #each in #424, nothing is rendered until resolved. There's only one problem with async content - since a new view is created each time, the value may flicker. I decided to leave it for now and look into it later.

---

Note that async spread attributes, e.g., <img {{attributes}}> are NOT supported yet.

[StorytellerCZ]

LGTM

[DanielDornhardt]

Hi guys! This is important stuff for templates forward compatibility with async / prepping code for Async / Meteor 3.0 in Meteor 2.x. Any chance that this'll be merged soon & possibly be included in the 2.14 release?

It's this PR and this one: Added support for Promises in #if and #each which would be important I think.

The previous one [https://github.com//pull/424](Added support for Promises in #if and #each) we're running in our fairly big meteor blaze app for some weeks now & it looks like it has no bad side effects on existing code as well as doing what it's supposed to do.

This one I could try to give a shake this week as well in our codebase.

What do you think @StorytellerCZ @Grubba27 @radekmie ?

[StorytellerCZ]

@jankapunkt @radekmie I leave this to you guys to figure out if it is ready to merge. I'm happy to get all the async stuff merged and release an alpha of Blaze 3.

[Grubba27]

lgtm

[radekmie]

@Grubba27 This PR was supposed to be merged later 🙈

[Grubba27]

Sorry 😢 I thought we were merging those PRs.
Would like me to revert those changes?

[radekmie]

No, it's all good. I just wanted to merge them in the opposite order, so their diffs would be meaningful - now this one includes the other one.