Revert "Remove NullPointerException."

sdk/lib/_internal/compiler/compiler.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.
 
 library compiler;
 
 import 'dart:uri';
 import 'implementation/apiimpl.dart';
 
 // Unless explicitly allowed, passing [:null:] for any argument to the
-// methods of library will result in an Error being thrown.
+// methods of library will result in a NullPointerException being
+// thrown.
 
 /**
  * Returns a future that completes to the source corresponding to
  * [uri]. If an exception occurs, the future completes with this
  * exception.
  */
 typedef Future<String> ReadStringFromUri(Uri uri);
 
 /**
  * Invoked by the compiler to report diagnostics. If [uri] is
@@ skipping 95 matching lines @@
   final String name;
 
   /**
    * This constructor is not private to support user-defined
    * diagnostic kinds.
    */
   const Diagnostic(this.ordinal, this.name);
 
   String toString() => name;
 }