Clone on mount

This is the first step towards descriptors. This will start cloning the
component when it's mounted instead of mounting the first instance.

This avoids an issue where a reference to the first instance can hang around
in props. Since a mounted component gets mutated, the descriptor changes.

We don't need to clone the props object itself. Mutating the shallow props
object of a child that's passed into you is already flawed. Those cases need to
use cloneWithProps. A props object is considered shallow frozen after it leaves
the render it was created in.

src/utils/traverseAllChildren.js

@@ -126,7 +126,8 @@ var traverseAllChildrenImpl =
         // All of the above are perceived as null.
         callback(traverseContext, null, storageName, indexSoFar);
         subtreeCount = 1;
-      } else if (children.mountComponentIntoNode) {
+      } else if (children.type && children.type.prototype &&
+                 children.type.prototype.mountComponentIntoNode) {
         callback(traverseContext, children, storageName, indexSoFar);
         subtreeCount = 1;
       } else {