Use static final empty Modifier instead of null.

lib/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.
 
 // Returns null if no need to rename a node.
 typedef String Renamer(Node node);
 
 String unparse(Node node) {
   Unparser unparser = new Unparser();
   unparser.unparse(node);
@@ skipping 106 matching lines @@
     sb.add(')');
     visit(node.body);
   }
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     visit(node.function);
   }
 
   visitFunctionExpression(FunctionExpression node) {
     // Check length to not print unnecessary whitespace.
-    if (node.modifiers !== null && node.modifiers.nodes.length() > 0) {
+    if (node.modifiers.nodes.length() > 0) {
       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 371 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }