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

runtime/vm/flow_graph_builder.cc

 // 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.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/compiler.h"
@@ skipping 1241 matching lines @@
                                           const String& dst_name) {
   ASSERT(!dst_type.IsNull());
   ASSERT(dst_type.IsFinalized());
 
   // If the destination type is malformed or malbounded, a dynamic type error
   // must be thrown at run time.
   if (dst_type.IsMalformedOrMalbounded()) {
     return false;
   }
 
-  // Any type is more specific than the dynamic type, the Object type, or void.
-  if (dst_type.IsDynamicType() || dst_type.IsObjectType() ||
-      dst_type.IsVoidType()) {
+  // Any type is more specific than the dynamic type and than the Object type.
+  if (dst_type.IsDynamicType() || dst_type.IsObjectType()) {
     return true;
   }
 
   // Do not perform type check elimination if this optimization is turned off.
   if (!FLAG_eliminate_type_checks) {
     return false;
   }
 
   // If nothing is known about the value, as is the case for passed-in
   // parameters, and since dst_type is not one of the tested cases above, then
@@ skipping 3119 matching lines @@
   ASSERT(type.HasResolvedTypeClass());
   const Class& type_class = Class::Handle(type.type_class());
   // Bail if the type has any type parameters.
   if (type_class.IsGeneric()) return false;
 
   // Finally a simple class for instance of checking.
   return true;
 }
 
 }  // namespace dart