Change ReactID.ATTR_NAME to "data-reactid"

This final change is what we've all been waiting for.

Note that it no longer makes sense to use `document.getElementById` in
`getNode`, because that only ever worked with "id" attributes.

src/core/ReactID.js

@@ -21,7 +21,7 @@
 
 var invariant = require('invariant');
 var ReactMount = require('ReactMount');
-var ATTR_NAME = 'id';
+var ATTR_NAME = 'data-reactid';
 var nodeCache = {};
 
 /**
@@ -97,7 +97,6 @@ function getNode(id) {
   }
 
   return nodeCache[id] =
-    document.getElementById(id) || // TODO Quit using getElementById.
     ReactMount.findReactRenderedDOMNodeSlow(id);
 }
 

src/core/ReactMount.js

@@ -55,10 +55,9 @@ function getReactRootID(container) {
  *
  *   ReactMount.renderComponent(component, $('container'));
  *
- *   TODO Update this comment when ReactID.ATTR_NAME changes.
- *   <div id="container">         <-- Supplied `container`.
- *     <div id=".reactRoot[3]">   <-- Rendered reactRoot of React component.
- *       // ...
+ *   <div id="container">                   <-- Supplied `container`.
+ *     <div data-reactid=".reactRoot[3]">   <-- Rendered reactRoot of React
+ *       // ...                                 component.
  *     </div>
  *   </div>
  *