Thread and Timeline fixes for Mojo.

runtime/vm/os_thread.h

Index: runtime/vm/os_thread.h
diff --git a/runtime/vm/os_thread.h b/runtime/vm/os_thread.h
index 9dbe50bd647208799aeec9318e2f8067a0de57b1..b831bd93fafd13f850acb59455adbba05eae8fea 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -58,7 +58,7 @@ class OSThread : public BaseThread {
     return id_;
   }
 
-  ThreadId join_id() const {
+  ThreadJoinId join_id() const {
     ASSERT(join_id_ != OSThread::kInvalidThreadJoinId);
     return join_id_;
   }
@@ -126,6 +126,8 @@ class OSThread : public BaseThread {
         Thread* vm_thread = reinterpret_cast<Thread*>(thread);
         os_thread = GetOSThreadFromThread(vm_thread);
       }
+    } else {
+      os_thread = CreateAndSetUnknownThread();
     }
     return os_thread;
   }
@@ -171,7 +173,7 @@ class OSThread : public BaseThread {
   // Called at VM startup and shutdown.
   static void InitOnce();
 
-  static bool IsThreadInList(ThreadId join_id);
+  static bool IsThreadInList(ThreadJoinId join_id);
 
   static const intptr_t kStackSizeBuffer = (4 * KB * kWordSize);
 
@@ -197,11 +199,12 @@ class OSThread : public BaseThread {
   static OSThread* GetOSThreadFromThread(Thread* thread);
   static void AddThreadToList(OSThread* thread);
   static void RemoveThreadFromList(OSThread* thread);
+  static OSThread* CreateAndSetUnknownThread();
 
   static ThreadLocalKey thread_key_;
 
   const ThreadId id_;
-  const ThreadId join_id_;
+  const ThreadJoinId join_id_;
   const ThreadId trace_id_;  // Used to interface with tracing tools.
   char* name_;  // A name for this thread.