Merge IO v2 branch to bleeding edge

samples/buildhook1/build.dart

Index: samples/buildhook1/build.dart
diff --git a/samples/buildhook1/build.dart b/samples/buildhook1/build.dart
index ca25ad5eb01cce6fbe53ab369842c9589cced784..5c912265bcf7eb9bfbc06c59895b139acd60248a 100644
--- a/samples/buildhook1/build.dart
+++ b/samples/buildhook1/build.dart
@@ -25,8 +25,8 @@ 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();
   }
 }