VM thread shutdown.

runtime/vm/os_thread_win.cc

Index: runtime/vm/os_thread_win.cc
diff --git a/runtime/vm/os_thread_win.cc b/runtime/vm/os_thread_win.cc
index 126594910b55bc0cae7d1169ee3b59f25039a100..90106e6a55ee4cda2a7e2b132814fd12ad0803af 100644
--- a/runtime/vm/os_thread_win.cc
+++ b/runtime/vm/os_thread_win.cc
@@ -71,6 +71,7 @@ int OSThread::Start(ThreadStartFunction function, uword parameter) {
 
 ThreadLocalKey OSThread::kUnsetThreadLocalKey = TLS_OUT_OF_INDEXES;
 ThreadId OSThread::kInvalidThreadId = 0;
+ThreadJoinId OSThread::kInvalidThreadJoinId = 0;
 
 ThreadLocalKey OSThread::CreateThreadLocal(ThreadDestructor unused) {
   ThreadLocalKey key = TlsAlloc();
@@ -101,14 +102,17 @@ ThreadId OSThread::GetCurrentThreadId() {
 }
 
 
-bool OSThread::Join(ThreadId id) {
+ThreadJoinId OSThread::GetCurrentThreadJoinId() {
+  return ::GetCurrentThreadId();
+}
+
+
+void OSThread::Join(ThreadJoinId id) {
   HANDLE handle = OpenThread(SYNCHRONIZE, false, id);
-  if (handle == INVALID_HANDLE_VALUE) {
-    return false;
-  }
+  ASSERT(handle != INVALID_HANDLE_VALUE);
   DWORD res = WaitForSingleObject(handle, INFINITE);
   CloseHandle(handle);
-  return res == WAIT_OBJECT_0;
+  ASSERT(res == WAIT_OBJECT_0);
 }