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));
+}