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

runtime/vm/compiler.h

Index: runtime/vm/compiler.h
diff --git a/runtime/vm/compiler.h b/runtime/vm/compiler.h
index ce7fdb8ac3223e87a15bc948ab73c4fb8c3de976..d226ccb92a9255002309e1510925e2e9e3bba77d 100644
--- a/runtime/vm/compiler.h
+++ b/runtime/vm/compiler.h
@@ -146,7 +146,8 @@ class Compiler : public AllStatic {
   // because the mutator thread changed the state (e.g., deoptimization,
   // deferred loading). The background compilation may retry to compile
   // the same function later.
-  static void AbortBackgroundCompilation(intptr_t deopt_id);
+  static void AbortBackgroundCompilation(intptr_t deopt_id,
+                                         const char* msg = "");
 };