Document that IPv4 are preferred in socket.connect.

Bug: #50868
Change-Id: I8186d95d1a24c77c4cfade533af26695c7a13b24
CoreLibraryReviewExempt: documentation-only change
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/293020
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

CHANGELOG.md

@@ -172,14 +172,20 @@
 
 #### `dart:io`
 
-- Deprecated `NetworkInterface.listSupported`. Has always returned true since
+- Deprecate `NetworkInterface.listSupported`. Has always returned true since
   Dart 2.3.
 - Finalize `httpEnableTimelineLogging` parameter name transition from `enable`
   to `enabled`. See [#43638][].
+- Favor IPv4 connections over IPv6 when connecting sockets. See
+  [#50868].
 - **Breaking change** [#51035][]:
   - Update `NetworkProfiling` to accommodate new `String` ids
     that are introduced in vm_service:11.0.0
 
+[#43638]: https://github.com/dart-lang/sdk/issues/43638
+[#50868]: https://github.com/dart-lang/sdk/issues/50868
+[#51035]: https://github.com/dart-lang/sdk/issues/51035
+
 #### `dart:js_util`
 
 - Added several helper functions to access more JavaScript operator, like

sdk/lib/io/socket.dart

@@ -549,9 +549,10 @@ abstract class RawSocket implements Stream<RawSocketEvent> {
   ///
   /// The [host] can either be a [String] or an [InternetAddress]. If [host] is a
   /// [String], [connect] will perform a [InternetAddress.lookup] and try
-  /// all returned [InternetAddress]es, until connected. Unless a
-  /// connection was established, the error from the first failing connection is
-  /// returned.
+  /// all returned [InternetAddress]es, until connected. If IPv4 and IPv6
+  /// addresses are both availble then connections over IPv4 are preferred. If
+  /// no connection can be establed then the error from the first failing
+  /// connection is returned.
   ///
   /// The argument [sourceAddress] can be used to specify the local
   /// address to bind when making the connection. The [sourceAddress] can either