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

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index f42555ebb34e0c015373e2e3c4fbc92a96be958e..383cd5b2ebc8a2287fccbbabf42f9b0005e41509 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -455,14 +455,14 @@ class FileIOException implements Exception {
                          OSError this.osError = null]);
   String toString() {
     StringBuffer sb = new StringBuffer();
-    sb.add("FileIOException");
+    sb.write("FileIOException");
     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();
   }