Implement sun.misc.Unsafe and fix the jfieldID/jmethodID leak.

Change-Id: I2997e19d9ae5e332a6731e7e10ebeff32bff085a
diff --git a/src/heap.h b/src/heap.h
index 1c2b7c9..8ffb558 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -140,7 +140,10 @@
   // Must be called if a field of an Object in the heap changes, and before any GC safe-point.
   // The call is not needed if NULL is stored in the field.
   static void WriteBarrier(const Object* object) {
-    // TODO: re-enable card marking when concurrent collector is active.
+#ifdef CONCURRENT_GARBAGE_COLLECTOR
+    // TODO: we need card marking for a concurrent collector.
+    UNIMPLEMENTED(FATAL);
+#endif
   }
 
  private: