Allow accessing siblings by ref

This makes it possible to write a wider range of components without
depending on internal implementations.

src/core/ReactComponent.js

@@ -564,6 +564,22 @@ var ReactComponent = {
      */
     isOwnedBy: function(owner) {
       return this.props[OWNER] === owner;
+    },
+
+    /**
+     * Gets another component, that shares the same owner as this one, by ref.
+     *
+     * @param {string} ref of a sibling Component.
+     * @return {?ReactComponent} the actual sibling Component.
+     * @final
+     * @internal
+     */
+    getSiblingByRef: function(ref) {
+      var owner = this.props[OWNER];
+      if (!owner || !owner.refs) {
+        return null;
+      }
+      return owner.refs[ref];
     }
 
   }