Follow-up to #3963. (Returning an Error wasn't useful; it just caused a later error when actually using it because type checkers need to be functions.)
This heuristic isn't great because it relies on inspecting deep children which aren't guaranteed to be React elements. In particular, this was causing stack overflows in a component we had that used a *DOM node* as children, like `<DOMContainer>{node}</DOMContainer>`.
This reverts commits:
0a3aa8493a64c9d9d7620c58f4f6b18cf226e442086636747f
The new folder structure is organized around major packages that are expected to ship separately in some form.
`/isomorphic`
I moved classic/modern and children utils into a directory called "isomorphic" with the main export being ReactIsomorphic. This will eventually become the "react" package.
This includes all the dependencies that you might need to create a component without dependencies on the renderer/reconciler.
The rest moves into decoupled renderers.
`/renderers/dom/client` - This is the main renderer for DOM.
`/renderers/dom/server` - This is the server-side renderer for HTML strings.
`/addons` and `/test` - Same as before for now.
You're not supposed to take on a dependency inside another package.
Shared code is organized into a "shared" directory which is intended to support all the packages in that subdirectory. Meaning that once we swap to CommonJS modules, the only time you should use `..` is to target `../shared/` or `../../shared`.
E.g. `/shared/` is common utils that are used by everything.
`/renderers/shared/` is code that is shared by all renderers, such as the main reconciliation algorithm.
Shared code will likely be copied into each package rather than referenced. This allow us to have separate state and allow inlining and deadcode elimination.
Size comparison:
```
raw gz Compared to master @ 6ed98ec0c8
= = build/JSXTransformer.js
-15736 -3247 build/react-with-addons.js
+287 +7 build/react-with-addons.min.js
-14412 -2887 build/react.js
+274 +15 build/react.min.js
```
Differences mostly look to be various bits of whitespace that Babel ends up removing during its transforms (https://gist.github.com/spicyj/21ef31f4d95fb7a58daf). In minified files, mostly additions of `"use strict";`.