Roll Observatory packages and add a roll script

packages/yaml/test.dart

+import 'package:yaml/yaml.dart';
+
+main() {
+  const src = """
+name: linter
+version: 0.0.1
+author: Dart Team <misc@dartlang.org>
+authors:
+  - Bill
+  - Ted
+description: Style linter for Dart.
+documentation:
+homepage: https://github.com/dart-lang/linter
+dependencies:
+  transmogrify:
+    hosted:
+      name: transmogrify
+      url: http://your-package-server.com
+    version: '>=0.4.0 <1.0.0'
+  analyzer: '0.24.0-dev.1'
+  cli_util: '>=0.0.1 <0.1.0'
+  semver: '>=0.2.0 <0.3.0'
+  yaml: '>=2.1.2 <3.0.0'
+  kittens:
+    git:
+      url: git://github.com/munificent/kittens.git
+      ref: some-branch
+  foo: any
+dev_dependencies:
+  markdown: '>=0.7.1+2 <0.8.0'
+  unittest: '>=0.11.0 <0.12.0'
+""";
+
+  YamlMap node = loadYamlNode(src, sourceUrl: null);
+  node.nodes.forEach((k, v) {
+    if (k is YamlScalar) print(k.span);
+  });
+}