Rewrite the pkg/yaml parser.

pkg/string_scanner/lib/src/string_scanner.dart

 // Copyright (c) 2014, 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 string_scanner.string_scanner;
 
 import 'package:source_span/source_span.dart';
 
 import 'exception.dart';
 import 'utils.dart';
@@ skipping 113 matching lines @@
 
   /// Returns whether or not [pattern] matches at the current position of the
   /// string.
   ///
   /// This doesn't move the scan pointer forward.
   bool matches(Pattern pattern) {
     _lastMatch = pattern.matchAsPrefix(string, position);
     return _lastMatch != null;
   }
 
+  /// Returns the substring of [string] between [start] and [end].
+  ///
+  /// Unlike [String.substring], [end] defaults to [position] rather than the
+  /// end of the string.
+  String substring(int start, [int end]) {
+    if (end == null) end = position;
+    return string.substring(start, end);
+  }
+
   /// Throws a [FormatException] with [message] as well as a detailed
   /// description of the location of the error in the string.
   ///
   /// [match] is the match information for the span of the string with which the
   /// error is associated. This should be a match returned by this scanner's
   /// [lastMatch] property. By default, the error is associated with the last
   /// match.
   ///
   /// If [position] and/or [length] are passed, they are used as the error span
   /// instead. If only [length] is passed, [position] defaults to the current
@@ skipping 14 matching lines @@
     throw new StringScannerException(message, span, string);
   }
 
   // TODO(nweiz): Make this handle long lines more gracefully.
   /// Throws a [FormatException] describing that [name] is expected at the
   /// current position in the string.
   void _fail(String name) {
     error("expected $name.", position: this.position, length: 0);
   }
 }