Fix warnings in socket.dart.

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 9152beebda0ea1b0457906ca209e4f44b0b52ade..a759598def0bc35786ffb83ada29ac917d265fdc 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -763,7 +763,11 @@ class SocketException implements IOException {
   final int port;
 
   const SocketException(this.message, {this.osError, this.address, this.port});
-  const SocketException.closed() : message = 'Socket has been closed';
+  const SocketException.closed()
+      : message = 'Socket has been closed',
+        osError = null,
+        address = null,
+        port = null;
 
   String toString() {
     StringBuffer sb = new StringBuffer();