Bring the YAML package's style up to modern standards.

pkg/yaml/README.md

 A parser for [YAML](http://www.yaml.org/).
 
 Use `loadYaml` to load a single document, or `loadYamlStream` to load a
 stream of documents. For example:
 
-    import 'package:yaml/yaml.dart';
-    main() {
-      var doc = loadYaml("YAML: YAML Ain't Markup Language");
-      print(doc['YAML']);
-    }
+```dart
+import 'package:yaml/yaml.dart';
+
+main() {
+  var doc = loadYaml("YAML: YAML Ain't Markup Language");
+  print(doc['YAML']);
+}
+```
 
 This library currently doesn't support dumping to YAML. You should use
 `JSON.encode` from `dart:convert` instead:
 
-    import 'dart:convert';
-    import 'package:yaml/yaml.dart';
-    main() {
-      var doc = loadYaml("YAML: YAML Ain't Markup Language");
-      print(JSON.encode(doc));
-    }
+```dart
+import 'dart:convert';
+import 'package:yaml/yaml.dart';
+
+main() {
+  var doc = loadYaml("YAML: YAML Ain't Markup Language");
+  print(JSON.encode(doc));
+}
+```
 
 The source code for this package is at <http://code.google.com/p/dart>.
 Please file issues at <http://dartbug.com>. Other questions or comments can be
 directed to the Dart mailing list at <mailto:misc@dartlang.org>.