Add lock owner information for class Monitor and add assertions for wait/notify/notifyall.

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 5ac2a2776b0a302f782d0c2d8b2e6daa1b4ce850..5929ef4503ff1aed8bf3f1fc79373d9323428e02 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -113,8 +113,22 @@ class Monitor {
   void Notify();
   void NotifyAll();
 
+#if defined(DEBUG)
+  bool IsOwnedByCurrentThread() const {
+    return owner_ == OSThread::GetCurrentThreadId();
+  }
+#else
+  bool IsOwnedByCurrentThread() const {
+    UNREACHABLE();
+    return false;
+  }
+#endif
+
  private:
   MonitorData data_;  // OS-specific data.
+#if defined(DEBUG)
+  ThreadId owner_;
+#endif  // defined(DEBUG)
 
   DISALLOW_COPY_AND_ASSIGN(Monitor);
 };