Restore About.GWT_VERSION_NUM for 2.6.0 release.

Fixes "unknown version" warning from old versions of GPE.

Bug: issue 8430
Change-Id: I9eebc13c2a397c98d38e1327514272a6c5f547e0
diff --git a/dev/core/src/com/google/gwt/dev/About.java b/dev/core/src/com/google/gwt/dev/About.java
index 52a3e20..fee497f 100644
--- a/dev/core/src/com/google/gwt/dev/About.java
+++ b/dev/core/src/com/google/gwt/dev/About.java
@@ -24,6 +24,13 @@
  */
 public class About {
   /**
+   * @deprecated use {@link #getGwtVersionObject()} or
+   *             {@link #getGwtVersionNum()} instead.
+   */
+  @Deprecated
+  public static String GWT_VERSION_NUM;
+
+  /**
    * Tag used for text replacement of the SVN version (split up to avoid
    * replacing it here).
    */ 
@@ -64,6 +71,9 @@
     } else {
       gwtVersion = new GwtVersion(tmp);
     }
+
+    // Initialize deprecated constant
+    GWT_VERSION_NUM = getGwtVersionNum();
   }
 
   /**