Update dart2js unit tests to new library API.

dart/sdk/lib/_internal/compiler/implementation/tree/unparser.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 tree;
 
 String unparse(Node node) {
   Unparser unparser = new Unparser();
   unparser.unparse(node);
   return unparser.result;
@@ skipping 120 matching lines @@
           sb.add(' ');
         }
       }
       visit(send.selector);
     } else {
       visit(name);
     }
   }
 
   visitFunctionExpression(FunctionExpression node) {
-    // Check length to not print unnecessary whitespace.
-    if (node.modifiers.nodes.length > 0) {
+    if (!node.modifiers.nodes.isEmpty) {
       visit(node.modifiers);
       sb.add(' ');
     }
     if (node.returnType != null) {
       visit(node.returnType);
       sb.add(' ');
     }
     if (node.getOrSet != null) {
       add(node.getOrSet.value);
       sb.add(' ');
@@ skipping 174 matching lines @@
   visitTypeVariable(TypeVariable node) {
     visit(node.name);
     if (node.bound != null) {
       sb.add(' extends ');
       visit(node.bound);
     }
   }
 
   visitVariableDefinitions(VariableDefinitions node) {
     visit(node.modifiers);
-    if (node.modifiers.nodes.length > 0) {
+    if (!node.modifiers.nodes.isEmpty) {
       sb.add(' ');
     }
     if (node.type != null) {
       visit(node.type);
       sb.add(' ');
     }
     visit(node.definitions);
     if (node.endToken.value == const SourceString(';')) {
       add(node.endToken.value);
     }
@@ skipping 249 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }