Add -Xcheck:jni.

I think this is as complete as possible right now. The remaining
two #if 0 sections require:

1. a way to get the Method* of the current native method.
2. a way to get the Class* of the type of a given Field*.

Change-Id: I331586022095fb36ccc10c9ac1890a59a9224d01
diff --git a/src/heap.h b/src/heap.h
index 1e56778..bbd05db 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -33,9 +33,14 @@
   // Allocates and initializes storage for an object instance.
   static Object* AllocObject(Class* klass, size_t num_bytes);
 
-  // Check sanity of given reference
+  // Check sanity of given reference. Requires the heap lock.
   static void VerifyObject(Object *obj);
 
+  // A weaker test than VerifyObject that doesn't require the heap lock,
+  // and doesn't abort on error, allowing the caller to report more
+  // meaningful diagnostics.
+  static bool IsHeapAddress(Object* obj);
+
   // Initiates an explicit garbage collection.
   static void CollectGarbage();