Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/lib/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.
 
 // This is a copy of the VM's dart:io library. This API is not usable
 // when running inside a web browser. Nevertheless, Leg provides a
 // mock version of the dart:io library so that it can statically
 // analyze programs that use dart:io.
 
 // TODO(ahe): Separate API from implementation details.
@@ skipping 50 matching lines @@
 class _Platform implements Platform {
   factory Platform() {
     throw new UnsupportedOperationException('new Platform()');
   }
 }
 
 class _Directory implements Directory {
   factory Directory(arg) {
     throw new UnsupportedOperationException('new Directory($arg)');
   }
+
+  factory Directory.current() {
+    throw new UnsupportedOperationException('new Directory.current()');
+  }
 }
 
 class _Process implements Process {
   factory Process.start(String executable,
                         List<String> arguments,
                         [String workingDirectory]) {
     var msg = 'new Process.start($executable, $arguments, $workingDirectory';
     throw new UnsupportedOperationException(msg);
   }
 }
 
 class _ServerSocket implements ServerSocket {
   factory _ServerSocket(String bindAddress, int port, int backlog) {
     throw new UnsupportedOperationException(
         'new ServerSocket($bindAddress, $port, $backlog)');
   }
 }
 
 class _Socket implements Socket {
   factory Socket(String host, int port) {
-    throw new UnsupportedOperationException('new Socket($host, $int)');
+    throw new UnsupportedOperationException('new Socket($host, $port)');
   }
 }
 
 class _EventHandler {
   factory _EventHandler() {
     throw new UnsupportedOperationException('new _EventHandler()');
   }
 
   static void _start() {
     throw new UnsupportedOperationException('_EventHandler._start()');
   }
 
   static _sendData(int id, ReceivePort receivePort, int data) {
     var msg = '_EventHandler._sendData($id, $receivePort, $data)';
     throw new UnsupportedOperationException(msg);
   }
 
   static _EventHandler get _eventHandler() {
     throw new UnsupportedOperationException('_EventHandler._eventhandler');
   }
 
   static void set _eventHandler(_EventHandler e) {
     throw new UnsupportedOperationException('_EventHandler._eventhandler = $e');
   }
 }