Use static final empty Modifier instead of null.

lib/compiler/implementation/tree/nodes.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.
 
 abstract class Visitor<R> {
   R visitBlock(Block node);
   R visitBreakStatement(BreakStatement node);
   R visitCascade(Cascade node);
   R visitCascadeReceiver(CascadeReceiver node);
   R visitCaseMatch(CaseMatch node);
@@ skipping 591 matching lines @@
   final NodeList parameters;
 
   final Statement body;
   final TypeAnnotation returnType;
   final Modifiers modifiers;
   final NodeList initializers;
 
   final Token getOrSet;
 
   FunctionExpression(this.name, this.parameters, this.body, this.returnType,
-                     this.modifiers, this.initializers, this.getOrSet);
+                     this.modifiers, this.initializers, this.getOrSet) {
+    assert(modifiers !== null);
+  }
 
   FunctionExpression asFunctionExpression() => this;
 
   accept(Visitor visitor) => visitor.visitFunctionExpression(this);
 
   visitChildren(Visitor visitor) {
     if (modifiers !== null) modifiers.accept(visitor);
     if (returnType !== null) returnType.accept(visitor);
     if (name !== null) name.accept(visitor);
     if (parameters !== null) parameters.accept(visitor);
@@ skipping 364 matching lines @@
     return (bound !== null) ? bound.getEndToken() : name.getEndToken();
   }
 }
 
 class VariableDefinitions extends Statement {
   final Token endToken;
   final TypeAnnotation type;
   final Modifiers modifiers;
   final NodeList definitions;
   VariableDefinitions(this.type, this.modifiers, this.definitions,
-                      this.endToken);
+                      this.endToken) {
+    assert(modifiers !== null);
+  }
 
   VariableDefinitions asVariableDefinitions() => this;
 
   accept(Visitor visitor) => visitor.visitVariableDefinitions(this);
 
   visitChildren(Visitor visitor) {
     if (type !== null) type.accept(visitor);
     if (definitions !== null) definitions.accept(visitor);
   }
 
@@ skipping 78 matching lines @@
     if (expression !== null) expression.accept(visitor);
   }
 
   Token getBeginToken() => beginToken;
 
   Token getEndToken() => beginToken.endGroup;
 }
 
 /** Representation of modifiers such as static, abstract, final, etc. */
 class Modifiers extends Node {
+  /**
+   * Pseudo-constant for empty modifiers. Use this instead of null.

[floitsch, 2012/10/01 09:02:59]

Remove "Use this instead of null".

[Johnni Winther, 2012/10/01 09:12:11]

Why?

+   */
+  static final Modifiers EMPTY = new Modifiers(new NodeList.empty());
+
   /* TODO(ahe): The following should be validated relating to modifiers:
    * 1. The nodes must come in a certain order.
    * 2. The keywords "var" and "final" may not be used at the same time.
    * 3. The keywords "abstract" and "external" may not be used at the same time.
    * 4. The type of an element must be null if isVar() is true.
    */
 
   final NodeList nodes;
   /** Bit pattern to easy check what modifiers are present. */
   final int flags;
 
   static const int FLAG_STATIC = 1;
   static const int FLAG_ABSTRACT = FLAG_STATIC << 1;
   static const int FLAG_FINAL = FLAG_ABSTRACT << 1;
   static const int FLAG_VAR = FLAG_FINAL << 1;
   static const int FLAG_CONST = FLAG_VAR << 1;
   static const int FLAG_FACTORY = FLAG_CONST << 1;
   static const int FLAG_EXTERNAL = FLAG_FACTORY << 1;
 
   Modifiers(NodeList nodes) : this.withFlags(nodes, computeFlags(nodes.nodes));
 
-  Modifiers.empty() : this(new NodeList.empty());
-
   Modifiers.withFlags(this.nodes, this.flags);
 
   static int computeFlags(Link<Node> nodes) {
     int flags = 0;
     for (; !nodes.isEmpty(); nodes = nodes.tail) {
       String value = nodes.head.asIdentifier().source.stringValue;
       if (value === 'static') flags |= FLAG_STATIC;
       else if (value === 'abstract') flags |= FLAG_ABSTRACT;
       else if (value === 'final') flags |= FLAG_FINAL;
       else if (value === 'var') flags |= FLAG_VAR;
@@ skipping 764 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send !== null) return f(send.arguments.head);
 }