Fix assertion in hosted mode for GlobalKey.dialog

We must test isShowing here, not isVisible, as the popup may be added
to the DOM tree, slated for display, but not yet visible as the code
is still trying to compute the correct size and location before the
popup is made visible to the user.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/src/main/java/com/google/gwtexpui/globalkey/client/GlobalKey.java b/src/main/java/com/google/gwtexpui/globalkey/client/GlobalKey.java
index b99e63f..16c2970 100644
--- a/src/main/java/com/google/gwtexpui/globalkey/client/GlobalKey.java
+++ b/src/main/java/com/google/gwtexpui/globalkey/client/GlobalKey.java
@@ -83,7 +83,7 @@
   public static void dialog(final PopupPanel panel) {
     initEvents();
     initDialog();
-    assert panel.isVisible();
+    assert panel.isShowing();
     assert active == global;
     active = new State(panel);
     active.add(new KeyCommand(0, KeyCodes.KEY_ESCAPE, Util.C