Roll to pickup pool changes

packages/csslib/lib/src/css_printer.dart

 // Copyright (c) 2013, 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;
 
 /**
  * Visitor that produces a formatted string representation of the CSS tree.
  */
 class CssPrinter extends Visitor {
@@ skipping 511 matching lines @@
   void visitOperatorPlus(OperatorPlus node) {
     emit('+');
   }
 
   void visitOperatorMinus(OperatorMinus node) {
     emit('-');
   }
 
   void visitVarUsage(VarUsage node) {
     emit('var(${node.name}');
-    if (!node.defaultValues.isEmpty) {
+    if (node.defaultValues.isNotEmpty) {
       emit(',');
       for (var defaultValue in node.defaultValues) {
         emit(' ');
         defaultValue.visit(this);
       }
     }
     emit(')');
   }
 
   void visitExpressions(Expressions node) {
@@ skipping 27 matching lines @@
 
   void visitWildcard(Wildcard node) {
     emit('*');
   }
 
   void visitDartStyleExpression(DartStyleExpression node) {
     // TODO(terry): TBD
     throw UnimplementedError;
   }
 }