Got rid of peculiar null literal in parsing an incomplete new expression.

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

Index: compiler/java/com/google/dart/compiler/ast/DartExprStmt.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartExprStmt.java b/compiler/java/com/google/dart/compiler/ast/DartExprStmt.java
index 2cc10ca4c7c72965d0e670d13b2a33f27fb4625d..6bf272baaf7000ea6b34aa539f27b4013cc93bd3 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartExprStmt.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartExprStmt.java
@@ -1,4 +1,4 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
@@ -21,7 +21,9 @@ public class DartExprStmt extends DartStatement {
 
   @Override
   public void visitChildren(ASTVisitor<?> visitor) {
-    expr.accept(visitor);
+    if (expr != null) {
+      expr.accept(visitor);
+    }
   }
 
   @Override