Fixed version.dart to use Future.immediate

We use a checked-in binary of dart that doesn't have Future.value().

BUG=http://dartbug.com/10193

Review URL: https://codereview.chromium.org//14146019

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@22010 260f80e4-7a28-3924-810f-c04153c831b5

tools/release/version.dart

@@ -149,7 +149,7 @@ class Version {
 
   Future<int> getRevision() {
     if (repositoryType == RepositoryType.UNKNOWN) {
-      return new Future.value(0);
+      return new Future.immediate(0);
     }
     var isSvn = repositoryType == RepositoryType.SVN;
     var command = isSvn ? "svn" : "git";