RemoveFromGraph on definitions asserts empty use lists.

runtime/vm/flow_graph_allocator.cc

Index: runtime/vm/flow_graph_allocator.cc
diff --git a/runtime/vm/flow_graph_allocator.cc b/runtime/vm/flow_graph_allocator.cc
index 69fc5c619d249789208d38832a74a46306a61c7d..47577f6a612812307eb6a030baa4411fc8a70238 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -104,6 +104,9 @@ void FlowGraphAllocator::EliminateEnvironmentUses() {
 
           PushArgumentInstr* push_argument = def->AsPushArgument();
           if ((push_argument != NULL) && push_argument->WasEliminated()) {
+            // TODO(zerny): This should be unreachable if we could properly
+            // replace uses by values. (See RemovePushArguments in
+            // flow_graph_optimizer.cc).
             (*values)[i] = push_argument->value();
             continue;
           }