Add Null class to dart:core.

sdk/lib/_internal/lib/js_helper.dart

Index: sdk/lib/_internal/lib/js_helper.dart
diff --git a/sdk/lib/_internal/lib/js_helper.dart b/sdk/lib/_internal/lib/js_helper.dart
index ab69f61f4867d2b92b91bce4047777f8b7ca5a39..52d2cb5dbadba0668f1abef7510fe9e92e9e1ce5 100644
--- a/sdk/lib/_internal/lib/js_helper.dart
+++ b/sdk/lib/_internal/lib/js_helper.dart
@@ -23,6 +23,7 @@ import 'dart:_foreign_helper' show DART_CLOSURE_TO_JS,
                                    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,
@@ -1611,18 +1612,6 @@ class JSName {
 }
 
 /**
- * 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.