Fix ReactEmptyComponent disappearing and throwing in IE8

[syranide]

One possible fix for #1494 and at the same time enabling the safe use of non-visible tags in IE8 for React components as well.

The down-side is cost, http://jsperf.com/testnoscrip, in worst-case scenarios it's 20% slower, that is; there's no leading text and only one element being inserted. However, using a more precise test (with no hits) the overall performance seems to only dip by 6%. Looking at http://jsperf.com/testnoscrip/3 with 6 elements being inserted, the cost of the lookups has already become reduced to only 2% slower overall performance. This does NOT include any overhead by React nor layouting/rendering, so the numbers should be significantly lower in practice. Also, this obviously only affects IE8.

Another possibility is to tag HTML insertions of non-visible tags inside React which could probably remove the overhead all-together, but I don't think there's an obvious non-intrusive way of doing it in React that would be worth it considering only IE8 is affected.

My PR implements the faster fix, including another check intended to further improve certain cases.

   raw     gz Compared to master @ 32b84a4c5ea32835b93a857cf00f0db86d6c755a
     =      = build/JSXTransformer-previous.js
     =      = build/JSXTransformer.js
     =      = build/react-previous.min.js
 -1477   +141 build/react-test.js
  +569   +184 build/react-with-addons.js
  +148    +45 build/react-with-addons.min.js
  +569   +181 build/react.js
  +148    +46 build/react.min.js

[syranide]

cc @chenglou @zpao

[chenglou]

that's neat lol, 👍

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[zpao]

@chenglou can you get the right people looking at this and get it reviewed and landed?

[syranide]

@chenglou Rebased, no conflicts. I don't know if there are any practical implications, but using <link> or <meta> instead would have the benefit of being inherently self-closing tags (not that it really matters in practice size-wise).

[chenglou]

@syranide diff created, although I can't repro this. I also vaguely remember being able to repro it, weird...

[syranide]

@chenglou It happens if there is no text before the "invisible" tag. You can try it with just plain innerHTML to see the effect.

[chenglou]

Reproed! Merged internally.