Restore HTMLPanel.add(Widget, Element) as public

Lots of users (at least within Google) rely on this method being
public.  Not sure why api-check didn't notice this would be a breaking
change.

Change-Id: I04152036a651f05f15201e822ac7bdb025c7e7e1

user/src/com/google/gwt/user/client/ui/HTMLPanel.java

diff --git a/user/src/com/google/gwt/user/client/ui/HTMLPanel.java b/user/src/com/google/gwt/user/client/ui/HTMLPanel.java
index 995a16b..c05044b 100644
--- a/user/src/com/google/gwt/user/client/ui/HTMLPanel.java
+++ b/user/src/com/google/gwt/user/client/ui/HTMLPanel.java
@@ -167,6 +167,19 @@
   }
 
   /**
+   * Adds a child widget to the panel, contained within an HTML
+   * element.  It is up to the caller to ensure that the given element
+   * is a child of this panel's root element.
+   *
+   * @param widget the widget to be added
+   * @param elem the element within which it will be contained
+   */
+  @Override
+  public void add(Widget widget, Element elem) {
+    super.add(widget, elem);
+  }
+
+  /**
    * Adds a child widget to the panel, replacing the HTML element.
    *
    * @param widget the widget to be added