Add forward-declarations in attempt to fix Mac/Windows build.

runtime/bin/thread.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_THREAD_H_
 #define BIN_THREAD_H_
 
 #include "platform/globals.h"
 
+namespace dart {
+namespace bin {
+class Thread;
+class Mutex;
+class Monitor;
+}
+}
+
 // Declare the OS-specific types ahead of defining the generic classes.
 #if defined(TARGET_OS_ANDROID)
 #include "bin/thread_android.h"
 #elif defined(TARGET_OS_LINUX)
 #include "bin/thread_linux.h"
 #elif defined(TARGET_OS_MACOS)
 #include "bin/thread_macos.h"
 #elif defined(TARGET_OS_WINDOWS)
 #include "bin/thread_win.h"
 #else
@@ skipping 76 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(Monitor);
 };
 
 
 }  // namespace bin
 }  // namespace dart
 
 
 #endif  // BIN_THREAD_H_