Revert "Void is not required to be `null` anymore."

tests/language/void_check_test.dart

-// Copyright (c) 2017, 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.
-
-// Tests that `void` accepts any value and won't throw on non-`null` values.
-// The test is set up in a way that `--trust-type-annotations` and type
-// propagation must not assume that `void` is `null` either.
-
-import 'package:expect/expect.dart';
-
-class A {
-  void foo() {
-    return bar();
-  }
-
-  void bar() {}
-}
-
-class B extends A {
-  int bar() => 42;
-}
-
-// Makes the typing cleaner: the return type here is `dynamic` and we are
-// guaranteed that there won't be any warnings.
-// Dart2js can still infer the type by itself.
-@NoInline()
-callFoo(A a) => a.foo();
-
-main() {
-  var a = new A();
-  var b = new B();
-  // The following line is not throwing, even though `a.foo()` (inside
-  // `callFoo`) is supposedly `void`.
-  callFoo(b).abs();
-  Expect.isNull(callFoo(a));
-  Expect.equals(42, callFoo(b));
-}