Fixing invalid code in docs for HttpRequest constructor.

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 583 matching lines @@
           "   */"
         ],
         "XMLHttpRequest": [
           "/**",
           "   * General constructor for any type of request (GET, POST, etc).",
           "   *",
           "   * This call is used in conjunction with [open]:",
           "   *",
           "   *     var request = new HttpRequest();",
           "   *     request.open('GET', 'http://dartlang.org')",
-          "   *     request.onLoad.add((event) => print('Request complete'));",
+          "   *     request.onLoad.listen((event) => print('Request complete'));",
           "   *",
           "   * is the (more verbose) equivalent of",
           "   *",
           "   *     HttpRequest.getString('http://dartlang.org').then(",
           "   *         (result) => print('Request complete: $result'));",
           "   */"
         ]
       }
     }
   },
@@ skipping 11 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }