Print messages when aborting background compilation (--trace-compiler); turn on background compilat…

runtime/vm/block_scheduler.cc

Index: runtime/vm/block_scheduler.cc
diff --git a/runtime/vm/block_scheduler.cc b/runtime/vm/block_scheduler.cc
index bc9885dcecf9e2ae463c8a6a7af18bba3273ac6b..9c00cfbe81879c7b706f150c351fe2eecffe370f 100644
--- a/runtime/vm/block_scheduler.cc
+++ b/runtime/vm/block_scheduler.cc
@@ -61,7 +61,8 @@ void BlockScheduler::AssignEdgeWeights() const {
       flow_graph()->parsed_function().function().ic_data_array());
   if (Compiler::IsBackgroundCompilation() && ic_data_array.IsNull()) {
     // Deferred loading cleared ic_data_array.
-    Compiler::AbortBackgroundCompilation(Thread::kNoDeoptId);
+    Compiler::AbortBackgroundCompilation(Thread::kNoDeoptId,
+        "BlockScheduler: ICData array cleared");
   }
   ASSERT(!ic_data_array.IsNull());
   Array& edge_counters = Array::Handle();