Remove unmodifiable map wrappers that could instead use UnmodifiableMapView.

sdk/lib/io/io.dart

 // Copyright (c) 2012, 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.
 
 /**
  * File, socket, HTTP, and other I/O support for server applications.
  *
  * The I/O library is used for Dart server applications,
  * which run on a stand-alone Dart VM from the command line.
  * *This library does not work in browser-based applications.*
@@ skipping 183 matching lines @@
  * [tutorial about writing command-line apps]
  * (https://www.dartlang.org/docs/tutorials/io/).
  */
 library dart.io;
 
 import 'dart:async';
 import 'dart:_internal';
 import 'dart:collection' show HashMap,
                               HashSet,
                               LinkedList,
-                              LinkedListEntry;
+                              LinkedListEntry,
+                              UnmodifiableMapView;
 import 'dart:convert';
 import 'dart:isolate';
 import 'dart:math';
 import 'dart:typed_data';
 
 part 'bytes_builder.dart';
 part 'common.dart';
 part 'crypto.dart';
 part 'data_transformer.dart';
 part 'directory.dart';
@@ skipping 15 matching lines @@
 part 'platform_impl.dart';
 part 'process.dart';
 part 'socket.dart';
 part 'stdio.dart';
 part 'string_transformer.dart';
 part 'timer_impl.dart';
 part 'secure_socket.dart';
 part 'secure_server_socket.dart';
 part 'websocket.dart';
 part 'websocket_impl.dart';