Rewrite the pkg/yaml parser.

pkg/yaml/lib/src/utils.dart

Index: pkg/yaml/lib/src/utils.dart
diff --git a/pkg/yaml/lib/src/utils.dart b/pkg/yaml/lib/src/utils.dart
index 84c1113a0b5a830e9d05426a5c581c10dbae9e1c..445221fe4e93dba3f2ea5867f778da0703a8c846 100644
--- a/pkg/yaml/lib/src/utils.dart
+++ b/pkg/yaml/lib/src/utils.dart
@@ -4,6 +4,8 @@
 
 library yaml.utils;
 
+import 'package:source_span/source_span.dart';
+
 /// A pair of values.
 class Pair<E, F> {
   final E first;
@@ -13,3 +15,30 @@ class Pair<E, F> {
 
   String toString() => '($first, $last)';
 }
+
+/// Print a warning.
+///
+/// If [span] is passed, associates the warning with that span.
+void warn(String message, [SourceSpan span]) =>
+    yamlWarningCallback(message, span);
+
+/// A callback for emitting a warning.
+///
+/// [message] is the text of the warning. If [span] is passed, it's the portion
+/// of the document that the warning is associated with and should be included
+/// in the printed warning.
+typedef YamlWarningCallback(String message, [SourceSpan span]);
+
+/// A callback for emitting a warning.
+///
+/// In a very few cases, the YAML spec indicates that an implementation should
+/// emit a warning. To do so, it calls this callback. The default implementation
+/// prints a message using [print].
+YamlWarningCallback yamlWarningCallback = (message, [span]) {
+  // TODO(nweiz): Print to stderr with color when issue 6943 is fixed and
+  // dart:io is available.
+  if (span != null) message = span.message(message);
+  print(message);
+};
+
+