Support for Null as bottom in dart2js

tests/language/null_is_bottom_test.dart

+// Copyright (c) 2016, 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.
+
+// Test that Null is a subtype of any other type.
+
+import 'package:expect/expect.dart';
+
+class A {}
+typedef A ReturnA();
+typedef TakeA(A a);
+typedef Null ReturnNull();
+typedef TakeNull(Null n);
+
+@NoInline()
+testA(A a) {}
+
+@NoInline()
+testListA(List<A> list) {}
+
+@NoInline()
+testIsListA(var a) => a is List<A>;
+
+@NoInline()
+testAsListA(var a) => a as List<A>;
+
+@NoInline()
+testNull(Null n) {}
+
+@NoInline()
+testListNull(List<Null> list) {}
+
+@NoInline()
+testIsListNull(var a) => a is List<Null>;
+
+@NoInline()
+testAsListNull(var a) => a as List<Null>;
+
+@NoInline()
+testReturnA(ReturnA f) {}
+
+@NoInline()
+testIsReturnA(var f) => f is ReturnA;
+
+@NoInline()
+testAsReturnA(var f) => f as ReturnA;
+
+@NoInline()
+testReturnNull(ReturnNull f) {}
+
+@NoInline()
+testIsReturnNull(var f) => f is ReturnNull;
+
+@NoInline()
+testAsReturnNull(var f) => f as ReturnNull;
+
+@NoInline()
+testTakeA(TakeA f) {}
+
+@NoInline()
+testIsTakeA(var f) => f is TakeA;
+
+@NoInline()
+testAsTakeA(var f) => f as TakeA;
+
+@NoInline()
+testTakeNull(TakeNull f) {}
+
+@NoInline()
+testIsTakeNull(var f) => f is TakeNull;
+
+@NoInline()
+testAsTakeNull(var f) => f as TakeNull;
+
+Null returnNullFunc() => null;
+takeNullFunc(Null n) {}
+A returnAFunc() => null;
+takeAFunc(A a) {}
+
+main() {
+  var n = null;
+  var listNull = new List<Null>();
+  var a = new A();
+  var listA = new List<A>();
+
+  testA(n);                                                          /// 01: ok
+  testA(a);                                                          /// 02: ok
+  testListA(listNull);                                               /// 03: ok
+  testListA(listA);                                                  /// 04: ok
+  Expect.isTrue(testIsListA(listNull));                              /// 05: ok
+  Expect.isTrue(testIsListA(listA));                                 /// 06: ok
+  testAsListA(listNull);                                             /// 07: ok
+  testAsListA(listA);                                                /// 08: ok
+
+  testNull(n);                                                       /// 09: ok
+  testNull(a);                                       /// 10: dynamic type error
+  testListNull(listNull);                                            /// 11: ok
+  testListNull(listA);                               /// 12: dynamic type error
+  Expect.isTrue(testIsListNull(listNull));                           /// 13: ok
+  Expect.isFalse(testIsListNull(listA));                             /// 14: ok
+  testAsListNull(listNull);                                          /// 15: ok
+  Expect.throws(() => testAsListNull(listA), (e) => e is CastError); /// 16: ok
+
+  var returnNull = returnNullFunc;
+  var takeNull = takeNullFunc;
+  var returnA = returnAFunc;
+  var takeA = takeAFunc;
+
+  testReturnA(returnA);                                              /// 17: ok
+  testReturnA(returnNull);                                           /// 18: ok
+  Expect.isTrue(testIsReturnA(returnA));                             /// 19: ok
+  Expect.isTrue(testIsReturnA(returnNull));                          /// 20: ok
+  testAsReturnA(returnA);                                            /// 21: ok
+  testAsReturnA(returnNull);                                         /// 22: ok
+
+  // This is not valid in strong-mode: ()->A <: ()->Null
+  testReturnNull(returnA);                                           /// 23: ok
+  testReturnNull(returnNull);                                        /// 24: ok
+  // This is not valid in strong-mode: ()->A <: ()->Null
+  Expect.isTrue(testIsReturnNull(returnA));                          /// 25: ok
+  Expect.isTrue(testIsReturnNull(returnNull));                       /// 26: ok
+  // This is not valid in strong-mode: ()->A <: ()->Null
+  testAsReturnNull(returnA);                                         /// 27: ok
+  testAsReturnNull(returnNull);                                      /// 28: ok
+
+  testTakeA(takeA);                                                  /// 29: ok
+  // This is not valid in strong-mode: (Null)-> <: (A)->
+  testTakeA(takeNull);                                               /// 30: ok
+  Expect.isTrue(testIsTakeA(takeA));                                 /// 31: ok
+  // This is not valid in strong-mode: (Null)-> <: (A)->
+  Expect.isTrue(testIsTakeA(takeNull));                              /// 32: ok
+  testAsTakeA(takeA);                                                /// 33: ok
+  // This is not valid in strong-mode: (Null)-> <: (A)->
+  testAsTakeA(takeNull);                                             /// 34: ok
+
+  testTakeNull(takeA);                                               /// 35: ok
+  testTakeNull(takeNull);                                            /// 36: ok
+  Expect.isTrue(testIsTakeNull(takeA));                              /// 37: ok
+  Expect.isTrue(testIsTakeNull(takeNull));                           /// 38: ok
+  testAsTakeNull(takeA);                                             /// 39: ok
+  testAsTakeNull(takeNull);                                          /// 40: ok
+}