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();
 
   /**
    * Safely convert a value to a [String] description.
    *
@@ skipping 32 matching lines @@
 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. */
-  const ArgumentError([this.message]);
+  ArgumentError([this.message]);
 
   String toString() {
     if (message != null) {
       return "Illegal argument(s): $message";
     }
     return "Illegal argument(s)";
   }
 }
 
 /**
@@ skipping 160 matching lines @@
 
 class OutOfMemoryError implements Error {
   const OutOfMemoryError();
   String toString() => "Out of Memory";
 }
 
 class StackOverflowError implements Error {
   const StackOverflowError();
   String toString() => "Stack Overflow";
 }