Add Null class to dart:core.

sdk/lib/_internal/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 'dart:collection';
 import 'dart:_foreign_helper' show DART_CLOSURE_TO_JS,
                                    JS,
                                    JS_CALL_IN_ISOLATE,
                                    JS_CURRENT_ISOLATE,
                                    JS_CURRENT_ISOLATE_CONTEXT,
                                    JS_DART_OBJECT_CONSTRUCTOR,
                                    JS_FUNCTION_CLASS_NAME,
                                    JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG,
                                    JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG,
                                    JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG,
                                    JS_FUNCTION_TYPE_RETURN_TYPE_TAG,
                                    JS_FUNCTION_TYPE_TAG,
                                    JS_FUNCTION_TYPE_VOID_RETURN_TAG,
                                    JS_GET_NAME,
                                    JS_GLOBAL_OBJECT,
                                    JS_HAS_EQUALS,
                                    JS_IS_INDEXABLE_FIELD_NAME,
                                    JS_OBJECT_CLASS_NAME,
+                                   JS_NULL_CLASS_NAME,
                                    JS_OPERATOR_AS_PREFIX,
                                    JS_OPERATOR_IS_PREFIX,
                                    JS_SIGNATURE_NAME,
                                    RAW_DART_FUNCTION_REF;
 import 'dart:_interceptors';
 import 'dart:_collection-dev' as _symbol_dev;
 import 'dart:_js_names' show mangledNames, mangledGlobalNames;
 
 part 'constant_map.dart';
 part 'native_helper.dart';
@@ skipping 1559 matching lines @@
  *       @JSName('title')
  *       String $dom_title;
  *     }
  */
 class JSName {
   final String name;
   const JSName(this.name);
 }
 
 /**
- * Represents the type of Null. The compiler treats this specially.
- * TODO(lrn): Null should be defined in core. It's a class, like int.
- * It just happens to act differently in assignability tests and,
- * like int, can't be extended or implemented.
- */
-class Null {
-  factory Null() {
-    throw new UnsupportedError('new Null()');
-  }
-}
-
-/**
  * The following methods are called by the runtime to implement
  * checked mode and casts. We specialize each primitive type (eg int, bool), and
  * use the compiler's convention to do is-checks on regular objects.
  */
 boolConversionCheck(value) {
   boolTypeCheck(value);
   assert(value != null);
   return value;
 }
 
@@ skipping 306 matching lines @@
 }
 
 /**
  * Error thrown when a runtime error occurs.
  */
 class RuntimeError extends Error {
   final message;
   RuntimeError(this.message);
   String toString() => "RuntimeError: $message";
 }