Fix some strong mode issues in the core libraries.

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 class _FileStream extends Stream<List<int>> {
@@ skipping 181 matching lines @@
           error(e, stackTrace);
         }
       }, onDone: () {
         completer.complete(_file);
       }, onError: error, cancelOnError: true);
     }).catchError(completer.completeError);
     return completer.future;
   }
 
   Future<File> close() =>
-      _openFuture.then<File>((openedFile) => openedFile.close());
+      _openFuture
+          .then((openedFile) => openedFile.close())
+          .then((_) => _file);
 }
 
 // Class for encapsulating the native implementation of files.
 class _File extends FileSystemEntity implements File {
   final String path;
 
   // Constructor for file.
   _File(this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
@@ skipping 336 matching lines @@
 
   Future<List<String>> readAsLines({Encoding encoding: UTF8}) =>
       readAsString(encoding: encoding).then(const LineSplitter().convert);
 
   List<String> readAsLinesSync({Encoding encoding: UTF8}) =>
       const LineSplitter().convert(readAsStringSync(encoding: encoding));
 
   Future<File> writeAsBytes(List<int> bytes,
       {FileMode mode: FileMode.WRITE, bool flush: false}) {
     return open(mode: mode).then((file) {
-      return file.writeFrom(bytes, 0, bytes.length).then((_) {
+      return file.writeFrom(bytes, 0, bytes.length).then<File>((_) {
         if (flush) return file.flush().then((_) => this);
         return this;
       }).whenComplete(file.close);
     });
   }
 
   void writeAsBytesSync(List<int> bytes,
       {FileMode mode: FileMode.WRITE, bool flush: false}) {
     RandomAccessFile opened = openSync(mode: mode);
     try {
@@ skipping 522 matching lines @@
   void _checkAvailable() {
     if (_asyncDispatched) {
       throw new FileSystemException(
           "An async operation is currently pending", path);
     }
     if (closed) {
       throw new FileSystemException("File closed", path);
     }
   }
 }