Throw a NullPointerException when throwing null.

lib/compiler/implementation/lib/js_helper.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('js_helper');
 
 #import('coreimpl.dart');
 
 #source('constant_map.dart');
 #source('native_helper.dart');
@@ skipping 739 matching lines @@
   }
 
   static double random() => JS('double', @'Math.random()');
 }
 
 /**
  * Called by generated code to capture the stacktrace before throwing
  * an exception.
  */
 captureStackTrace(ex) {
+  if (ex === null) ex = const NullPointerException();
   var jsError = JS('Object', @'new Error()');
   JS('void', @'#.dartException = #', jsError, ex);
   JS('void', @'''#.toString = #''', jsError,
      DART_CLOSURE_TO_JS(toStringWrapper));
   return jsError;
 }
 
 /**
  * This method is installed as JavaScript toString method on exception
  * objects in [captureStackTrace]. So JavaScript 'this' binds to an
@@ skipping 414 matching lines @@
   propertyTypeCastError(value, property);
 }
 
 /**
  * Special interface recognized by the compiler and implemented by DOM
  * objects that support integer indexing. This interface is not
  * visible to anyone, and is only injected into special libraries.
  */
 interface JavaScriptIndexingBehavior {
 }