Add _android files for building DartVM on Android

runtime/bin/socket.h

 // 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.
 
 #ifndef BIN_SOCKET_H_
 #define BIN_SOCKET_H_
 
 #include "bin/builtin.h"
 #include "bin/utils.h"
 
 #include "platform/globals.h"
 #include "platform/thread.h"
 // Declare the OS-specific types ahead of defining the generic class.
-#if defined(TARGET_OS_LINUX)
+#if defined(TARGET_OS_ANDROID)
+#include "bin/socket_android.h"
+#elif defined(TARGET_OS_LINUX)
 #include "bin/socket_linux.h"
 #elif defined(TARGET_OS_MACOS)
 #include "bin/socket_macos.h"
 #elif defined(TARGET_OS_WINDOWS)
 #include "bin/socket_win.h"
 #else
 #error Unknown target os.
 #endif
 
 
@@ skipping 37 matching lines @@
   static intptr_t Accept(intptr_t fd);
   static intptr_t CreateBindListen(const char* bindAddress,
                                    intptr_t port,
                                    intptr_t backlog);
 
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(ServerSocket);
 };
 
 #endif  // BIN_SOCKET_H_