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

sdk/lib/_internal/js_runtime/lib/js_helper.dart

 // Copyright (c) 2013, 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.
 
 library _js_helper;
 
 import 'dart:_js_embedded_names'
     show
         DEFERRED_LIBRARY_URIS,
         DEFERRED_LIBRARY_HASHES,
@@ skipping 3397 matching lines @@
   if (JS('bool', '#[#]', getInterceptor(value), property)) return value;
   propertyTypeError(value, property);
 }
 
 listSuperNativeTypeCast(value, property) {
   if (value is List || value == null) return value;
   if (JS('bool', '#[#]', getInterceptor(value), property)) return value;
   propertyTypeCastError(value, property);
 }
 
+voidTypeCheck(value) {
+  if (value == null) return value;
+  throw new TypeErrorImplementation(value, 'void');
+}
+
 extractFunctionTypeObjectFrom(o) {
   var interceptor = getInterceptor(o);
   var signatureName = JS_GET_NAME(JsGetName.SIGNATURE_NAME);
   return JS('bool', '# in #', signatureName, interceptor)
       ? JS('', '#[#]()', interceptor, signatureName)
       : null;
 }
 
 functionTypeTest(value, functionTypeRti) {
   if (value == null) return false;
@@ skipping 367 matching lines @@
 // unneeded code.
 class _UnreachableError extends AssertionError {
   _UnreachableError();
   String toString() => "Assertion failed: Reached dead code";
 }
 
 @NoInline()
 void assertUnreachable() {
   throw new _UnreachableError();
 }