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 664ce35ef01122f5978618a9d31f839fa5cbf383..7a8e8445a801a33308f3cf04013e0872c0509b7a 100644
--- a/sdk/lib/_internal/lib/js_helper.dart
+++ b/sdk/lib/_internal/lib/js_helper.dart
@@ -1514,18 +1514,6 @@ 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.