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

sdk/lib/io/base64.dart

Index: sdk/lib/io/base64.dart
diff --git a/sdk/lib/io/base64.dart b/sdk/lib/io/base64.dart
index 12c1e5f8367a87f96a782162093ebc91a84b1b3c..1469c8d1190236ab62e0e1dbdee360d51ce38a7d 100644
--- a/sdk/lib/io/base64.dart
+++ b/sdk/lib/io/base64.dart
@@ -51,9 +51,9 @@ class _Base64 {
     StringBuffer output = new StringBuffer();
     for (i = 0; i < characters.length; i++) {
       if (i > 0 && i % 76 == 0) {
-        output.add("\r\n");
+        output.write("\r\n");
       }
-      output.add(characters[i]);
+      output.write(characters[i]);
     }
     return output.toString();
   }