Added slow_assert macro and flag for slow development assertions in the VM.

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
diff --git a/runtime/vm/flow_graph.cc b/runtime/vm/flow_graph.cc
index d434d610ebed6c587bd311253424cd663142f8e5..8d9ccae7707e5122fcb5de13b044c0da809d877a 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -12,6 +12,7 @@
 
 namespace dart {
 
+DECLARE_FLAG(bool, slow_assert);
 DECLARE_FLAG(bool, trace_optimization);
 
 FlowGraph::FlowGraph(const FlowGraphBuilder& builder,
@@ -282,7 +283,7 @@ void FlowGraph::ComputeUseLists() {
     ClearUseLists((*graph_entry_->initial_definitions())[i]);
   }
   ComputeUseListsRecursive(graph_entry_);
-  DEBUG_ASSERT(ValidateUseLists());
+  SLOW_ASSERT(ValidateUseLists());
 }