ART: Add simple null alias tracking for lock counting

Null is the only literal for objects, and one may lock and unlock
on different registers containing null, which is still balanced.

Bug: 23502994
Change-Id: Ibfbf1b8c2aa7d1409e3e426988d2d15efe1f2d0d
diff --git a/test/088-monitor-verification/src/Main.java b/test/088-monitor-verification/src/Main.java
index d742b14..212c894 100644
--- a/test/088-monitor-verification/src/Main.java
+++ b/test/088-monitor-verification/src/Main.java
@@ -221,6 +221,8 @@
                 IllegalMonitorStateException.class);
         runTest("UnbalancedJoin", new Object[] { new Object(), new Object() }, null);
         runTest("UnbalancedStraight", new Object[] { new Object(), new Object() }, null);
+        runTest("NullLocks", new Object[] { false }, null);
+        runTest("NullLocks", new Object[] { true }, NullPointerException.class);
     }
 
     private static void runTest(String className, Object[] parameters, Class<?> excType) {