Fix two checked mode bugs introduced by 0.13.0.

lib/parser.dart

 // Copyright (c) 2012, 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 csslib.parser;
 
 import 'dart:math' as math;
 
 import 'package:source_span/source_span.dart';
 
@@ skipping 757 matching lines @@
    *
    *  @mixin IDENT [(args,...)] '{'
    *    [ruleset | property | directive]*
    *  '}'
    */
   MixinDefinition processMixin() {
     _next();
 
     var name = identifier();
 
-    var params = <VarDefinition>[];
+    var params = <TreeNode>[];
     // Any parameters?
     if (_maybeEat(TokenKind.LPAREN)) {
       var mustHaveParam = false;
       var keepGoing = true;
       while (keepGoing) {
         var varDef = processVariableOrDirective(mixinParameter: true);
         if (varDef is VarDefinitionDirective || varDef is VarDefinition) {
           params.add(varDef);
         } else if (mustHaveParam) {
           _warning("Expecting parameter", _makeSpan(_peekToken.span));
@@ skipping 1958 matching lines @@
 
     if (replace != null && result == null) {
       result = new StringBuffer(text.substring(0, i));
     }
 
     if (result != null) result.write(replace != null ? replace : text[i]);
   }
 
   return result == null ? text : result.toString();
 }