Fix deprecation warnings in dart:io. Now completely warning free.

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 aa05fdcbf48bb65123d3a59c20f101198ce3cbaa..3424fed03a5b345752acaa4793a879744815d9f0 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -216,14 +216,14 @@ class SocketIOException implements Exception {
                            OSError this.osError = null]);
   String toString() {
     StringBuffer sb = new StringBuffer();
-    sb.add("SocketIOException");
+    sb.write("SocketIOException");
     if (!message.isEmpty) {
-      sb.add(": $message");
+      sb.write(": $message");
       if (osError != null) {
-        sb.add(" ($osError)");
+        sb.write(" ($osError)");
       }
     } else if (osError != null) {
-      sb.add(": $osError");
+      sb.write(": $osError");
     }
     return sb.toString();
   }