Make Future._nullFuture be initialized synchronously.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 4f1df90488355237b72866d04786b65987af6d12..595489e890f4adb45498f6a66b00b9551ea629ae 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -135,7 +135,7 @@ abstract class FutureOr<T> {
  */
 abstract class Future<T> {
   // The `_nullFuture` is a completed Future with the value `null`.
-  static final _Future<Null> _nullFuture = new Future.value(null);
+  static final _Future<Null> _nullFuture = new _Future<Null>.value(null);
 
   /**
    * Creates a future containing the result of calling [computation]