Make isEmpty a getter.

lib/compiler/implementation/tree/prettyprint.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.
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
  * .dart file.
  */
 class PrettyPrinter implements Visitor {
 
   /** String used to represent one level of indent. */
   static const String INDENT = "  ";
 
   StringBuffer sb;
   Link<String> tagStack;
 
   PrettyPrinter() :
       sb = new StringBuffer(),
       tagStack = const Link<String>();
 
   void pushTag(String tag) {
     tagStack = tagStack.prepend(tag);
   }
 
   String popTag() {
-    assert(!tagStack.isEmpty());
+    assert(!tagStack.isEmpty);
     String tag = tagStack.head;
     tagStack = tagStack.tail;
     return tag;
   }
 
   /**
    * Adds given string to result string.
    */
   void add(SourceString string) {
     string.printOn(sb);
@@ skipping 422 matching lines @@
   }
 
   visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }