Fix some strong mode warnings in the parser.

lib/src/parser.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.parser;
 
 import 'package:source_span/source_span.dart';
 import 'package:string_scanner/string_scanner.dart';
 
 import 'event.dart';
@@ skipping 242 matching lines @@
 
     if (token is AliasToken) {
       _scanner.scan();
       _state = _states.removeLast();
       return new AliasEvent(token.span, token.name);
     }
 
     var anchor;
     var tagToken;
     var span = token.span.start.pointSpan();
-    parseAnchor() {
+    parseAnchor(token) {
       anchor = token.name;
       span = span.expand(token.span);
-      token = _scanner.advance();
+      return _scanner.advance();
     }
 
-    parseTag() {
+    parseTag(token) {
       tagToken = token;
       span = span.expand(token.span);
-      token = _scanner.advance();
+      return _scanner.advance();
     }
 
     if (token is AnchorToken) {
-      parseAnchor();
-      if (token is TagToken) parseTag();
+      token = parseAnchor(token);
+      if (token is TagToken) token = parseTag(token);
     } else if (token is TagToken) {
-      parseTag();
-      if (token is AnchorToken) parseAnchor();
+      token = parseTag(token);
+      if (token is AnchorToken) token = parseAnchor(token);
     }
 
     var tag;
     if (tagToken != null) {
       if (tagToken.handle == null) {
         tag = tagToken.suffix;
       } else {
         var tagDirective = _tagDirectives[tagToken.handle];
         if (tagDirective == null) {
           throw new YamlException("Undefined tag handle.", tagToken.span);
@@ skipping 516 matching lines @@
 
   /// Expect nothing.
   static const END = const _State("END");
 
   final String name;
 
   const _State(this.name);
 
   String toString() => name;
 }