Revert "Remove NullPointerException."

sdk/lib/core/errors.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.
 
 class Error {
   const Error();
 }
 
 /**
  * Error thrown by the runtime system when an assert statement fails.
  */
 class AssertionError implements Error {
 }
 
 /**
  * Error thrown by the runtime system when a type assertion fails.
  */
 class TypeError implements AssertionError {
 }
 
 /**
  * Error thrown by the runtime system when a cast operation fails.
  */
 class CastError implements Error {
 }
 
 /**
- * Error thrown when attempting to throw [:null:].
- */
-class NullThrownError implements Error {
-  const NullThrownError();
-  String toString() => "Throw of null.";
-}
-
-/**
  * Error thrown when a function is passed an unacceptable argument.
  */
 class ArgumentError implements Error {
   final message;
 
   /** The [message] describes the erroneous argument. */
-  ArgumentError([this.message]);
+  const ArgumentError([this.message]);
 
   String toString() {
     if (message != null) {
       return "Illegal argument(s): $message";
     }
     return "Illegal argument(s)";
   }
 }
 
 /**
@@ skipping 179 matching lines @@
 
 class OutOfMemoryError implements Error {
   const OutOfMemoryError();
   String toString() => "Out of Memory";
 }
 
 class StackOverflowError implements Error {
   const StackOverflowError();
   String toString() => "Stack Overflow";
 }