runat-1128988: Add event.once

I'm really surprised that this wasn't there already.

Signed-off-by: Joe Walker <[email protected]>

lib/gcli/util/util.js

@@ -160,6 +160,20 @@ exports.createEvent = function(name) {
     handlers = [];
   };
 
+  /**
+   * Fire an event just once using a promise.
+   */
+  event.once = function() {
+    return new Promise(function(resolve, reject) {
+      var handler = function(arg) {
+        event.remove(handler);
+        resolve(arg);
+      };
+
+      event.add(handler);
+    });
+  },
+
   /**
    * Temporarily prevent this event from firing.
    * @see resumeFire(ev)