Adjusted HttpRequest documentation to discourage using open and send in simple cases.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index fd31da0dffe415c5fb8bf0c701a0ced57ca856c3..34d5647ca2f9b6bda1f63923443e2edf39aa4dd4 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -13400,6 +13400,11 @@ class HttpRequest extends XmlHttpRequestEventTarget native "XMLHttpRequest" {
    *
    * Calling `open` again on a currently active request is equivalent to
    * calling `abort`.
+   *
+   * Note: Most simple HTTP requests can be accomplished using the [getString],
+   * [request], [requestCrossOrigin], or [postFormData] methods. Use of this
+   * `open` method is intended only for more complext HTTP requests where
+   * finer-grained control is needed.
    */
   @DomName('XMLHttpRequest.open')
   @DocsEditable()
@@ -13422,6 +13427,11 @@ class HttpRequest extends XmlHttpRequestEventTarget native "XMLHttpRequest" {
   /**
    * Send the request with any given `data`.
    *
+   * Note: Most simple HTTP requests can be accomplished using the [getString],
+   * [request], [requestCrossOrigin], or [postFormData] methods. Use of this
+   * `send` method is intended only for more complext HTTP requests where
+   * finer-grained control is needed.
+   *
    * See also:
    *
    *   * [send](https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest#send%28%29)