Refactor Future constructors.

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index 65f7adfb75586a616a00fd4efb56aed66726a9ea..95dfc80fb4672a5a31dad07125a29597ff8323e0 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -58,7 +58,7 @@ class _FileStream extends Stream<List<int>> {
       _openedFile = null;
       return closeFuture;
     } else {
-      return new Future.immediate(null);
+      return new Future.value();
     }
   }
 
@@ -106,7 +106,7 @@ class _FileStream extends Stream<List<int>> {
     if (_path != null) {
       openFuture = new File(_path).open(mode: FileMode.READ);
     } else {
-      openFuture = new Future.immediate(_File._openStdioSync(0));
+      openFuture = new Future.value(_File._openStdioSync(0));
     }
     openFuture
       .then((RandomAccessFile opened) {
@@ -158,7 +158,7 @@ class _FileStreamConsumer extends StreamConsumer<List<int>> {
 
   _FileStreamConsumer.fromStdio(int fd) {
     assert(1 <= fd && fd <= 2);
-    _openFuture = new Future.immediate(_File._openStdioSync(fd));
+    _openFuture = new Future.value(_File._openStdioSync(fd));
   }
 
   Future<File> consume(Stream<List<int>> stream) {
@@ -561,7 +561,7 @@ class _File extends _FileBase implements File {
       sink.close();
       return sink.done.then((_) => this);;
     } catch (e) {
-      return new Future.immediateError(e);
+      return new Future.error(e);
     }
   }