[React] Don't attach to document in ReactTestUtils.renderIntoDocument.

src/test/ReactTestUtils.js

@@ -43,7 +43,11 @@ function Event(suffix) {}
 var ReactTestUtils = {
   renderIntoDocument: function(instance) {
     var div = document.createElement('div');
-    document.documentElement.appendChild(div);
+    // None of our tests actually require attaching the container to the
+    // DOM, and doing so creates a mess that we rely on test isolation to
+    // clean up, so we're going to stop honoring the name of this method
+    // (and probably rename it eventually) if no problems arise.
+    // document.documentElement.appendChild(div);
     return React.renderComponent(instance, div);
   },