Fix this->window confusion in DevMode

dev/core/src/com/google/gwt/dev/shell/JavaScriptHost.java

Index: dev/core/src/com/google/gwt/dev/shell/JavaScriptHost.java
diff --git a/dev/core/src/com/google/gwt/dev/shell/JavaScriptHost.java b/dev/core/src/com/google/gwt/dev/shell/JavaScriptHost.java
index 17a1b5c33d474dd8cccd175b74dfe62db0886f7e..8f3ecadd91bd390dbccce85ebf0d575f74b88249 100644
--- a/dev/core/src/com/google/gwt/dev/shell/JavaScriptHost.java
+++ b/dev/core/src/com/google/gwt/dev/shell/JavaScriptHost.java
@@ -23,6 +23,19 @@ public class JavaScriptHost {
 
   private static ShellJavaScriptHost sHost;
 
+  /**
+   * Throws {@code NullPointerException} if {@code o} is null.
+   *<p>
+   * Used by {@code RewriteJsniMethods} to ensure JSO instance methods
+   * are not called against null instances.
+   */
+  public static Object checkNotNull(Object o) {
+    if (o == null) {
+      throw new NullPointerException("JSO instance was null");
+    }
+    return o;
+  }
+
   public static void exceptionCaught(Object exception) {
     sHost.exceptionCaught(exception);
   }