Fix a crashing yaml bug when parsing block scalars.

pkg/yaml/lib/src/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 yaml.scanner;
 
 import 'package:collection/collection.dart';
 import 'package:string_scanner/string_scanner.dart';
 import 'package:source_span/source_span.dart';
 
@@ skipping 1138 matching lines @@
     // needed.
     var pair = _scanBlockScalarBreaks(indent);
     indent = pair.first;
     var trailingBreaks = pair.last;
 
     // Scan the block scalar contents.
     var buffer = new StringBuffer();
     var leadingBreak = '';
     var leadingBlank = false;
     var trailingBlank = false;
-    var end = _scanner.position;
+    var end = _scanner.state;
     while (_scanner.column == indent && !_scanner.isDone) {
       // Check for a document indicator. libyaml doesn't do this, but the spec
       // mandates it. See example 9.5:
       // http://yaml.org/spec/1.2/spec.html#id2801606.
       if (_isDocumentIndicator) break;
 
       // We are at the beginning of a non-empty line.
 
       // Is there trailing whitespace?
       trailingBlank = _isBlank;
@@ skipping 12 matching lines @@
       buffer.write(trailingBreaks);
 
       // Is there leading whitespace?
       leadingBlank = _isBlank;
 
       var startPosition = _scanner.position;
       while (!_isBreakOrEnd) {
         _scanner.readChar();
       }
       buffer.write(_scanner.substring(startPosition));
-      end = _scanner.position;
+      end = _scanner.state;
 
       // libyaml always reads a line here, but this breaks on block scalars at
       // the end of the document that end without newlines. See example 8.1:
       // http://yaml.org/spec/1.2/spec.html#id2793888.
       if (!_scanner.isDone) leadingBreak = _readLine();
 
       // Eat the following indentation and spaces.
       var pair = _scanBlockScalarBreaks(indent);
       indent = pair.first;
       trailingBreaks = pair.last;
@@ skipping 455 matching lines @@
 
   /// All trailing whitespace is preserved.
   static const KEEP = const _Chomping("KEEP");
 
   final String name;
 
   const _Chomping(this.name);
 
   String toString() => name;
 }