Update assert to follow the spec.

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

Index: compiler/java/com/google/dart/compiler/ast/DartAssertion.java
===================================================================
--- compiler/java/com/google/dart/compiler/ast/DartAssertion.java	(revision 409)
+++ compiler/java/com/google/dart/compiler/ast/DartAssertion.java	(working copy)
@@ -9,11 +9,9 @@
  */
 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) {
@@ -24,21 +22,10 @@
     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);
   }
@@ -46,9 +33,6 @@
   @Override
   public void visitChildren(DartPlainVisitor<?> visitor) {
     expression.accept(visitor);
-    if (message != null) {
-      message.accept(visitor);
-    }
   }
 
   @Override