Rewrite the pkg/yaml parser.

pkg/yaml/lib/src/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library yaml.utils;
 
+import 'package:source_span/source_span.dart';
+
 /// A pair of values.
 class Pair<E, F> {
   final E first;
   final F last;
 
   Pair(this.first, this.last);
 
   String toString() => '($first, $last)';
 }
+
+/// Print a warning.
+///
+/// If [span] is passed, associates the warning with that span.
+void warn(String message, [SourceSpan 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);
+}