Trying to generate an ImageResource yields NPE, but not where it should, and
hey, look at that assert that you think won't really be called. I called it!

Add the resource name which is wicked helpful.

Review by: fabbott@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10591 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/user/src/com/google/gwt/resources/rg/ImageResourceGenerator.java b/user/src/com/google/gwt/resources/rg/ImageResourceGenerator.java
index 6d38fb6..4509439 100644
--- a/user/src/com/google/gwt/resources/rg/ImageResourceGenerator.java
+++ b/user/src/com/google/gwt/resources/rg/ImageResourceGenerator.java
@@ -471,7 +471,9 @@
     ImageResourceDeclaration image = new ImageResourceDeclaration(method);
     DisplayedImage bundle = getImage(image);
     ImageRect rect = bundle.getImageRect(image);
-    assert rect != null : "No ImageRect ever computed for " + name;
+    if (rect == null) {
+      throw new NullPointerException("No ImageRect ever computed for " + name);
+    }
 
     String[] urlExpressions =
         new String[] {bundle.getNormalContentsFieldName(), bundle.getRtlContentsFieldName()};