Fix JSON example.

It seems that previous versions of GWT allowed entry point classes not
implementing EntryPoint but having the onModuleLoad method, but 2.7.0
disallows it for production and SDM, classic dev mode works though.

Change-Id: I902d8193b59c580f1d5ed52f73e2f232d0baaabf
(cherry picked from commit f75a50d9e249986bb3da1f969b286f82002f33a6)
diff --git a/samples/json/src/com/google/gwt/sample/json/client/JSON.java b/samples/json/src/com/google/gwt/sample/json/client/JSON.java
index 8f48b0f..517d9e1 100644
--- a/samples/json/src/com/google/gwt/sample/json/client/JSON.java
+++ b/samples/json/src/com/google/gwt/sample/json/client/JSON.java
@@ -15,6 +15,7 @@
  */
 package com.google.gwt.sample.json.client;
 
+import com.google.gwt.core.client.EntryPoint;
 import com.google.gwt.core.client.GWT;
 import com.google.gwt.event.dom.client.ClickEvent;
 import com.google.gwt.event.dom.client.ClickHandler;
@@ -50,7 +51,7 @@
  * servlet with this example and simply have it hit the yahoo service and return
  * the results.
  */
-public class JSON {
+public class JSON implements EntryPoint {
   /**
    * Class for handling the response text associated with a request for a JSON
    * object.