RemoveFromGraph on definitions asserts empty use lists.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index b24028dc0a904457a6096e86baf3dc4adb390599..f65dc45f7b1c0430bfb1aa59e30b0bbe0abe8d1b 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -146,7 +146,14 @@ static bool HasOnlyTwoDouble(const ICData& ic_data) {
 static void RemovePushArguments(InstanceCallComp* comp) {
   // Remove original push arguments.
   for (intptr_t i = 0; i < comp->ArgumentCount(); ++i) {
-    comp->ArgumentAt(i)->RemoveFromGraph();
+    PushArgumentInstr* push = comp->ArgumentAt(i);
+    // TODO(zerny): Currently the register allocator replaces unused pushes with
+    // their definitions. To do so here, we need first to link uses to their
+    // instructions and input index. Here push->ReplaceUsesWith requires that
+    // push->value() is a UseVal.
+    // (See FlowGraphAllocator::EliminateEnvironmentUses).
+    push->set_use_list(NULL);
+    push->RemoveFromGraph();
   }
 }