Add support to specify the source address for socket connect

sdk/lib/io/socket.dart

Index: sdk/lib/io/socket.dart
diff --git a/sdk/lib/io/socket.dart b/sdk/lib/io/socket.dart
index b95163c5567765976e6eea3c6c7635e37e82c0f8..70c68a0ecf066dea8362755bcf7bc5236101da22 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -453,8 +453,13 @@ abstract class RawSocket implements Stream<RawSocketEvent> {
    * all returned [InternetAddress]es, until connected. Unless a
    * connection was established, 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. `sourceAddress` can either
+   * be a `String` or an `InternetAddress`. If a `String` is passed it must
+   * hold a numeric IP address.
    */
-  external static Future<RawSocket> connect(host, int port);
+  external static Future<RawSocket> connect(host, int port, {sourceAddress});
 
   /**
    * Returns the number of received and non-read bytes in the socket that
@@ -546,8 +551,13 @@ abstract class Socket implements Stream<List<int>>, IOSink {
    * all returned [InternetAddress]es, until connected. Unless a
    * connection was established, 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. `sourceAddress` can either
+   * be a `String` or an `InternetAddress`. If a `String` is passed it must
+   * hold a numeric IP address.
    */
-  external static Future<Socket> connect(host, int port);
+  external static Future<Socket> connect(host, int port, {sourceAddress});
 
   /**
    * Destroy the socket in both directions. Calling [destroy] will make the