Start implementing jdb "locals".

This lets us show the names and types of the locals, but all the values
will show up as 0/null. We're going to have to walk the whole stack and
take callee-save frames into account to do that right.

Change-Id: Ic6e115513b6e65ae7ed4b7274e70bc514e83190a
diff --git a/src/image_writer.h b/src/image_writer.h
index 35486b0..ec17ebf 100644
--- a/src/image_writer.h
+++ b/src/image_writer.h
@@ -40,7 +40,7 @@
   static void SetImageOffset(Object* object, size_t offset) {
     DCHECK(object != NULL);
     // should be no lock (but it might be forward referenced interned string)
-    DCHECK(object->monitor_ == 0 || object->IsString());
+    DCHECK(object->monitor_ == 0 || object->GetClass()->IsStringClass());
     DCHECK_NE(0U, offset);
     object->monitor_ = offset;
   }