Issue 28027. Move Null to the bottom in the Analyzer.

tests/language/null_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.
 // Dart test for testing the ternary operator.
 
 import "package:expect/expect.dart";
 
 // Test that `Null` acts like the bottom type - less than any other type.
 
 bool isCheckedMode = () { bool c = false; assert(c = true); return c; } ();
@@ skipping 95 matching lines @@
   Expect.isTrue(((int _) => null) is Fun<int, Null>);
 
   Null fun(int x) => null;
   Fun<Null, int> fun2 = fun;  // Safe assignment.
   if (fun2 is Fun<int, Null>) {
     // If int->Null is *subtype* of Null->int (which it should be),
     // then type promotion succeeds.
     // If type promotion succeeds, the static type is int->Null, otherwise
     // it's Null->int.
     fun2(42);  // Should not give a warning after type promotion.
-    fun2(null).abs();  /// 03: static type warning, runtime error
+    fun2(null).abs();  /// 03: runtime error
   }
 }
 
 class T1 {
   Null foo(int x) => null;
   int bar(Null x) => null;
   Null baz(int x) => null;
   int qux(Null x) => null;
 }
 
 class T2 extends T1 {
-  Null foo(Null x) => null;  /// 01: static type warning
+  Null foo(Null x) => null;
   Null bar(Null x) => null;
-  int baz(int x) => x;  /// 02: static type warning
+  int baz(int x) => x;
   int qux(int x) => x;
 }
 
 // Avoid "variable not used" warnings.
 use(x) {
   return identical(x, x);
 }