Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/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:
+
+```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:
+
+```dart
+import 'dart:convert';
+import 'package:yaml/yaml.dart';
+
+main() {
+  var doc = loadYaml("YAML: YAML Ain't Markup Language");
+  print(JSON.encode(doc));
+}
+```