Fix fromEnvironment when called from isolates.

tests/corelib/bool_from_environment_default_value_test.dart

Index: tests/corelib/bool_from_environment_default_value_test.dart
diff --git a/tests/corelib/bool_from_environment_default_value_test.dart b/tests/corelib/bool_from_environment_default_value_test.dart
index f41252238f300526aa5fed7a4f8460db149aea65..1095e0e0f2149945ab8592a5f9a20d10429da8a4 100644
--- a/tests/corelib/bool_from_environment_default_value_test.dart
+++ b/tests/corelib/bool_from_environment_default_value_test.dart
@@ -2,11 +2,21 @@
 // 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.
 
+import "dart:isolate";
+
 import "package:expect/expect.dart";
 
-main() {
+void test(port) {
   Expect.isFalse(const bool.fromEnvironment('NOT_FOUND'));
   Expect.isTrue(const bool.fromEnvironment('NOT_FOUND', defaultValue: true));
   Expect.isFalse(const bool.fromEnvironment('NOT_FOUND', defaultValue: false));
   Expect.isNull(const bool.fromEnvironment('NOT_FOUND', defaultValue: null));
+  if (port != null) port.send(null);
+}
+
+main() {
+  test(null);
+  var port = new ReceivePort();
+  Isolate.spawn(test, port.sendPort);
+  port.listen((_) => port.close());
 }