Expose the rendered children before they're actually mounted

Exposing the _renderedChildren property before all the children are fully
mounted. This allows us to debug a partially mounted tree when the debugger
has a breakpoint in the one of the mounting children.

This only has a functional difference in the case where mounting throws. This
will end up not mounting the component anyway. Any remounting shouldn't be
affected by this change.

src/core/ReactMultiChild.js

@@ -197,6 +197,7 @@ var ReactMultiChild = {
     mountChildren: function(children, transaction) {
       var mountImages = [];
       var index = 0;
+      this._renderedChildren = children;
       for (var name in children) {
         var child = children[name];
         if (children.hasOwnProperty(name) && child) {
@@ -213,7 +214,6 @@ var ReactMultiChild = {
           index++;
         }
       }
-      this._renderedChildren = children;
       return mountImages;
     },