Make TreeNode.visit return dynamic.

lib/src/tree_base.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.
 
 part of csslib.visitor;
 
 /**
  * The base type for all nodes in a CSS abstract syntax tree.
  */
 abstract class TreeNode {
   /** The source code this [TreeNode] represents. */
   final SourceSpan span;
 
   TreeNode(this.span);
 
   TreeNode clone();
 
   /** Classic double-dispatch visitor for implementing passes. */
-  void visit(VisitorBase visitor);
+  visit(VisitorBase visitor);
 
   /** A multiline string showing the node and its children. */
   String toDebugString() {
     var to = new TreeOutput();
     var tp = new _TreePrinter(to, true);
     this.visit(tp);
     return to.buf.toString();
   }
 }
 
@@ skipping 58 matching lines @@
           writeln('null');
         }
       }
       depth -= 1;
       writeln(']');
     }
   }
 
   String toString() => buf.toString();
 }