Support for Null as bottom in dart2js

sdk/lib/_internal/js_runtime/lib/js_helper.dart

 // Copyright (c) 2013, 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:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
     DEFERRED_LIBRARY_HASHES,
     GET_TYPE_FROM_NAME,
@@ skipping 136 matching lines @@
 /// Returns true if the given [type] is _the_ `Function` type.
 // TODO(floitsch): move this to foreign_helper.dart or similar.
 @ForceInline()
 bool isDartFunctionTypeRti(Object type) {
   return JS_BUILTIN('returns:bool;effects:none;depends:none',
                     JsBuiltin.isGivenTypeRti,
                     type,
                     JS_GET_NAME(JsGetName.FUNCTION_CLASS_TYPE_NAME));
 }
 
+/// Returns true if the given [type] is _the_ `Null` type.
+@ForceInline()
+bool isNullType(Object type) {
+  return JS_BUILTIN('returns:bool;effects:none;depends:none',
+      JsBuiltin.isGivenTypeRti,
+      type,
+      JS_GET_NAME(JsGetName.NULL_CLASS_TYPE_NAME));
+}
+
 /// Returns whether the given type is _the_ Dart Object type.
 // TODO(floitsch): move this to foreign_helper.dart or similar.
 @ForceInline()
 bool isDartObjectTypeRti(type) {
   return JS_BUILTIN('returns:bool;effects:none;depends:none',
                     JsBuiltin.isGivenTypeRti,
                     type,
                     JS_GET_NAME(JsGetName.OBJECT_CLASS_TYPE_NAME));
 }
 
@@ skipping 3893 matching lines @@
 // unneeded code.
 class _UnreachableError extends AssertionError {
   _UnreachableError();
   String toString() => "Assertion failed: Reached dead code";
 }
 
 @NoInline()
 void assertUnreachable() {
   throw new _UnreachableError();
 }