Make IOSink implement StringSink

tests/standalone/io/process_stdout_test.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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.
 //
 // Process test program to test process communication.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
@@ skipping 27 matching lines @@
         // We expect an extra character on windows due to carriage return.
         if (13 == buffer[dataSize - 1] && dataSize + 1 == received) {
           Expect.equals(13, buffer[dataSize - 1]);
           Expect.equals(10, buffer[dataSize]);
           buffer.removeLast();
         }
       }
     }
 
     process.stderr.listen((_) {});
-    process.stdin.add(data);
+    process.stdin.writeBytes(data);
     process.stdin.close();
     process.stdout.listen(readData);
   });
 }
 
 main() {
   // Run the test using the process_test binary.
   test(Process.start(getProcessTestFileName(),
                      const ["0", "1", "99", "0"]), 99);
 
   // Run the test using the dart binary with an echo script.
   // The test runner can be run from either the root or from runtime.
   var scriptFile = new File("tests/standalone/io/process_std_io_script.dart");
   if (!scriptFile.existsSync()) {
     scriptFile = new File("../tests/standalone/io/process_std_io_script.dart");
   }
   Expect.isTrue(scriptFile.existsSync());
   test(Process.start(new Options().executable, [scriptFile.path, "0"]), 0);
 }