Fix for issue 27567 (inconsistent handling of invalid -D options

tests/standalone/env_test.dart

 // Copyright (c) 2016, 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.
+// SharedOptions=-Dvar -D -D=var -Dvar=invalid -Dvar=valid -Dvar
 
 import "package:expect/expect.dart";
 
-import 'dart:async';
-
-Future/*<T>*/ foo/*<T>*/(/*=T*/ x) async => x;
-
-main() async {
-  Expect.equals(1, await foo/*<int>*/(1));
+main() {
+  Expect.equals('valid', const String.fromEnvironment('var'));
 }