Don't use regular expressions in Loader.

lib/yaml.dart

Index: lib/yaml.dart
diff --git a/lib/yaml.dart b/lib/yaml.dart
index 9af329a202eb098850b5ac92c88a433239d8dbf5..aa120ef9790fc52daa793888010bb1455896f813 100644
--- a/lib/yaml.dart
+++ b/lib/yaml.dart
@@ -53,7 +53,7 @@ YamlDocument loadYamlDocument(String yaml, {sourceUrl}) {
   var document = loader.load();
   if (document == null) {
     return new YamlDocument.internal(
-        new YamlScalar.internal(null, loader.span, ScalarStyle.ANY),
+        new YamlScalar.internalWithSpan(null, loader.span),
         loader.span, null, const []);
   }