Add Null class to dart:core.

tests/language/null_test.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 // Second dart test program.
 
 import "package:expect/expect.dart";
 
-class NullTest {
-  static int foo(var obj) {
+class BadInherit
+  extends Null     /// 01: compile-time error
+  implements Null  /// 02: compile-time error
+    {}
+
+class BadEquals {
+  int get hashCode => null.hashCode;
+  bool operator==(Object other) => other == null;
+}
+
+class NotCalled {
+  bool operator==(Object other) {
+    throw "SHOULD NOT GET HERE";
+  }
+}
+
+confuse(x) {
+  try { throw [x]; } catch (e) { return e[0]; }
+  return 42;
+}
+
+main() {
+  new BadInherit();  // Make sure class is referenced.
+
+  int foo(var obj) {
     Expect.equals(null, obj);
   }
 
-  static bool compareToNull(var value) {
+  bool compareToNull(var value) {
     return null == value;
   }
 
-  static bool compareWithNull(var value) {
+  bool compareWithNull(var value) {
     return value == null;
   }
 
-  static int testMain() {
-    var val = 1;
-    var obj = null;
+  var val = 1;
+  var obj = confuse(null);
+  var bad = new BadEquals();
 
-    Expect.equals(null, obj);
-    Expect.equals(null, null);
+  Expect.isTrue(identical(obj, null));
 
+  Expect.isTrue(null == null);
+  Expect.isTrue(null == obj);
+  Expect.isTrue(obj == null);
+  Expect.isTrue(obj == obj);
+
+  // Using  == null  or  null ==  will not call any equality method.
+  Expect.isFalse(new NotCalled() == null);
+  Expect.isFalse(null == new NotCalled());
+  Expect.isFalse(new NotCalled() == obj);
+  Expect.isFalse(obj == new NotCalled());
+
+  Expect.isFalse(null == false);
+  Expect.isFalse(null == 0);
+  Expect.isFalse(null == "");
+  Expect.isFalse(null == []);
+  Expect.isFalse(null == 0.0);
+  Expect.isFalse(null == -0.0);
+  Expect.isFalse(null == double.NAN);
+
+  const t1 = null == null;
+  const t2 = null == 0;
+  const t3 = false == null;
+  Expect.isTrue(t1);
+  Expect.isFalse(t2);
+  Expect.isFalse(t3);
+
+  Expect.isFalse(bad == null);
+  Expect.isFalse(null == bad);
+
+  foo(obj);
+  foo(null);
+
+  if (obj != null) {
+    foo(null);
+  } else {
     foo(obj);
-    foo(null);
+  }
 
-    if (obj != null) {
-      foo(null);
-    } else {
-      foo(obj);
-    }
+  Expect.isTrue(null is Null);
+  Expect.isTrue(obj is Null);
+  Expect.isTrue(null is Object);
+  Expect.isTrue(obj is Object);
+  Expect.isTrue(null is dynamic);
+  Expect.isTrue(obj is dynamic);
+  Expect.isFalse(obj is String);
+  Expect.isFalse(null is! Null);
+  Expect.isFalse(obj is! Null);
+  Expect.isFalse(null is! Object);
+  Expect.isFalse(obj is! Object);
+  Expect.isFalse(null is! dynamic);
+  Expect.isFalse(obj is! dynamic);
+  Expect.isTrue(obj is !String);
+  Expect.isFalse(0 is Null);  // It's just assignable.
 
-    Expect.isFalse(compareToNull(val));
-    Expect.isTrue(compareToNull(obj));
-    Expect.isFalse(compareWithNull(val));
-    Expect.isTrue(compareWithNull(obj));
-    Expect.isTrue(obj is Object);
-    Expect.isFalse(obj is String);
-    Expect.isTrue(obj is !String);
-    Expect.isFalse(obj is !Object);
-    Expect.isFalse(val is !Object);
+  Expect.equals("null", null.toString());
+  Expect.equals("null", "${null}");
+  Expect.equals(Null, null.runtimeType);
 
-    return 0;
-  }
+  Expect.isFalse(compareToNull(val));
+  Expect.isTrue(compareToNull(obj));
+  Expect.isFalse(compareWithNull(val));
+  Expect.isTrue(compareWithNull(obj));
 }
-
-
-main() {
-  NullTest.testMain();
-}