[Fuchsia] Implements the thread interrupter for CPU profiling.

runtime/vm/os_thread_fuchsia.h

Index: runtime/vm/os_thread_fuchsia.h
diff --git a/runtime/vm/os_thread_fuchsia.h b/runtime/vm/os_thread_fuchsia.h
index b9275f9299c596d8ba8ecb19c24a091b45963a5c..713e08ab00247b52ef70fd8a39a043212d2386b7 100644
--- a/runtime/vm/os_thread_fuchsia.h
+++ b/runtime/vm/os_thread_fuchsia.h
@@ -9,6 +9,7 @@
 #error Do not include os_thread_fuchsia.h directly; use os_thread.h instead.
 #endif
 
+#include <magenta/syscalls/object.h>
 #include <pthread.h>
 
 #include "platform/assert.h"
@@ -17,7 +18,7 @@
 namespace dart {
 
 typedef pthread_key_t ThreadLocalKey;
-typedef pthread_t ThreadId;
+typedef mx_koid_t ThreadId;
 typedef pthread_t ThreadJoinId;