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

tests/language/void_type_test.dart

 // Copyright (c) 2012, 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 type checks involving the void type.
 
 import "package:expect/expect.dart";
 
 isCheckedMode() {
   try {
     var i = 1;
@@ skipping 23 matching lines @@
 
 void f_dyn_1() {
   var x = 1;
   return x;
 }
 
 void f_f() {
   return f();
 }
 
-void test(int n, void func()) {
+void test(int n, void func(), bool must_get_error) {
   // Test as closure call.
   {
     bool got_type_error = false;
     try {
       var x = func();
     } on TypeError catch (error) {
       got_type_error = true;
     }
-    Expect.isFalse(got_type_error);
+    // Never a type error in production mode.
+    if (isCheckedMode()) {
+      Expect.isTrue(got_type_error == must_get_error);
+    } else {
+      Expect.isFalse(got_type_error);
+    }
   }
   // Test as direct call.
   {
     bool got_type_error = false;
     try {
       var x;
       switch (n) {
         case 0:
           x = f();
           break;
         case 1:
           x = f_null();
           break;
         case 2:
           x = f_1();
           break;
         case 3:
           x = f_dyn_null();
           break;
         case 4:
           x = f_dyn_1();
           break;
         case 5:
           x = f_f();
           break;
       }
     } on TypeError catch (error) {
       got_type_error = true;
     }
-    Expect.isFalse(got_type_error);
+    // Never a type error in production mode.
+    if (isCheckedMode()) {
+      Expect.isTrue(got_type_error == must_get_error);
+    } else {
+      Expect.isFalse(got_type_error);
+    }
   }
 }
 
 main() {
-  test(0, f);
-  test(1, f_null);
-  test(2, f_1);
-  test(3, f_dyn_null);
-  test(4, f_dyn_1);
-  test(5, f_f);
+  test(0, f, false);
+  test(1, f_null, false);
+  test(2, f_1, true);
+  test(3, f_dyn_null, false);
+  test(4, f_dyn_1, true);
+  test(5, f_f, false);
 }