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

runtime/bin/thread.h

Index: runtime/bin/thread.h
diff --git a/runtime/bin/thread.h b/runtime/bin/thread.h
index eb99e1039ad58e63246e6c1e674485fe3830b726..60dc75f24f465dafe62a6a7b0e051bd5fa88411e 100644
--- a/runtime/bin/thread.h
+++ b/runtime/bin/thread.h
@@ -7,6 +7,14 @@
 
 #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"