Merge IO v2 branch to bleeding edge

samples/buildhook1/build.dart

 // Copyright (c) 2012, 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.
 
 import "dart:io";
 
 /**
  * This minimal build script copies the contents of .foo files to .foobar files.
  * In order to be invoked automatically by the Editor, this script must be named
  * 'build.dart' and placed in the root of a project.
  */
 void main() {
   for (String arg in new Options().arguments) {
     if (arg.startsWith("--changed=")) {
       String file = arg.substring("--changed=".length);
 
       if (file.endsWith(".foo")) {
         _processFile(file);
       }
     }
   }
 }
 
 void _processFile(String file) {
   String contents = new File(file).readAsStringSync();
 
   if (contents != null) {
-    OutputStream out = new File("${file}bar").openOutputStream();
-    out.writeString("// processed from ${file}:\n${contents}");
+    var out = new File("${file}bar").openWrite();
+    out.addString("// processed from ${file}:\n${contents}");
     out.close();
   }
 }