Add detailed error message to socket_source_address_test.

tests/standalone/io/socket_source_address_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 
 import "dart:async";
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 Future throws(Function f, Function check) async {
   try {
     await f();
     Expect.fail('Did not throw');
   } catch (e) {
     if (check != null) {
       if (!check(e)) {
         Expect.fail('Unexpected: $e');
       }
     }
   }
 }
 
 Future testArguments(connectFunction) async {
+  var sourceAddress;
   asyncStart();
   var server = await ServerSocket.bind(InternetAddress.LOOPBACK_IP_V4, 0);
-  server.listen((_) { throw 'Connection unexpected'; },
-                onDone: () => asyncEnd());
+  server.listen((_) {
+    throw 'Unexpected connection from address $sourceAddress';
+  }, onDone: () => asyncEnd());
 
   asyncStart();
   // Illegal type for sourceAddress.
-  for (var sourceAddress in ['www.google.com', 'abc']) {
+  for (sourceAddress in ['www.google.com', 'abc']) {
     await throws(() => connectFunction('127.0.0.1',
                                        server.port,
                                        sourceAddress: sourceAddress),
                  (e) => e is ArgumentError);
   }
   // Unsupported local address.
-  for (var sourceAddress in ['8.8.8.8', new InternetAddress('8.8.8.8')]) {
+  for (sourceAddress in ['8.8.8.8', new InternetAddress('8.8.8.8')]) {
     await throws(() => connectFunction('127.0.0.1',
                                        server.port,
                                        sourceAddress: sourceAddress),
                  (e) => e is SocketException);
   }
   // Address family mismatch.
-  for (var sourceAddress in ['::1', InternetAddress.LOOPBACK_IP_V6]) {
+  for (sourceAddress in ['::1', InternetAddress.LOOPBACK_IP_V6]) {
     await throws(() => connectFunction('127.0.0.1',
                                        server.port,
                                        sourceAddress: sourceAddress),
                  (e) => e is SocketException);
   }
   asyncEnd();
   server.close();
 }
 
 // IPv4 addresses to use as source address when connecting locally.
@@ skipping 78 matching lines @@
       InternetAddress.ANY_IP_V4, false, Socket.connect, (s) => s.destroy());
   testConnect(
       InternetAddress.ANY_IP_V6, false, RawSocket.connect, (s) => s.close());
   testConnect(
       InternetAddress.ANY_IP_V6, false, Socket.connect, (s) => s.destroy());
   testConnect(
       InternetAddress.ANY_IP_V6, true, RawSocket.connect, (s) => s.close());
   testConnect(
       InternetAddress.ANY_IP_V6, true, Socket.connect, (s) => s.destroy());
 }