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..940175cdd6c0b9d32e8a18cfe5a686129d44fd8b 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -4,6 +4,7 @@
 
 #include "vm/flow_graph.h"
 
+#include "vm/assert.h"
 #include "vm/bit_vector.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/intermediate_language.h"
@@ -282,7 +283,7 @@ void FlowGraph::ComputeUseLists() {
     ClearUseLists((*graph_entry_->initial_definitions())[i]);
   }
   ComputeUseListsRecursive(graph_entry_);
-  DEBUG_ASSERT(ValidateUseLists());
+  SLOW_ASSERT(ValidateUseLists());
 }