Trick React into thinking a container is a class

Summary:
The factory component pattern is about to be deprecate in:

facebook/react#15145

This change makes it so that React will call this constructor with `new` as if
it is a class component instead of just calling it as a function component.

There might be a slight perf penalty to doing this my understanding is that this
is not the preferred path anyway.

Reviewed By: kassens

Differential Revision: D14516681

fbshipit-source-id: 751890a98639687b208633f2e4c8e71e5df60776

Author: sebmarkbage

packages/react-relay/buildReactRelayContainer.js

@@ -71,6 +71,7 @@ function buildReactRelayContainer<TBase: React$ComponentType<*>>(
     // $FlowFixMe
     return new Container(props);
   }
+  ContainerConstructor.prototype = React.Component.prototype;
 
   function forwardRef(props, ref) {
     const context = readContext(ReactRelayContext);