Update assert to follow the spec.

compiler/java/com/google/dart/compiler/ast/DartAssertion.java

 // Copyright (c) 2011, 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.
 
 package com.google.dart.compiler.ast;
 
 /**
  * Implements the "assert" statement.
  */
 public class DartAssertion extends DartStatement {
   private DartExpression expression;
-  private DartExpression message;
 
-  public DartAssertion(DartExpression expression, DartExpression message) {
+  public DartAssertion(DartExpression expression) {
     this.expression = becomeParentOf(expression);
-    this.message = becomeParentOf(message);
   }
 
   public void setExpression(DartExpression expression) {
     this.expression = expression;
   }
 
   public DartExpression getExpression() {
     return expression;
   }
 
-  public void setMessage(DartExpression message) {
-    this.message = message;
-  }
-
-  public DartExpression getMessage() {
-    return message;
-  }
-
   @Override
   public void traverse(DartVisitor v, DartContext ctx) {
     if (v.visit(this, ctx)) {
       expression = becomeParentOf(v.accept(expression));
-      if (message != null) {
-        message = becomeParentOf(v.accept(message));
-      }
     }
     v.endVisit(this, ctx);
   }
 
   @Override
   public void visitChildren(DartPlainVisitor<?> visitor) {
     expression.accept(visitor);
-    if (message != null) {
-      message.accept(visitor);
-    }
   }
 
   @Override
   public <R> R accept(DartPlainVisitor<R> visitor) {
     return visitor.visitAssertion(this);
   }
 }