Bailout if field state changed during background compilation.

runtime/vm/compiler.h

Index: runtime/vm/compiler.h
diff --git a/runtime/vm/compiler.h b/runtime/vm/compiler.h
index 4aa4249e348c644f3686f8c4803cc1eddb548f22..e193dbf856a212b72b13f5d1018980b4da5deba4 100644
--- a/runtime/vm/compiler.h
+++ b/runtime/vm/compiler.h
@@ -94,6 +94,12 @@ class Compiler : public AllStatic {
   static RawError* CompileAllFunctions(const Class& cls);
 
   static bool always_optimize();
+
+  // Notify the compiler that background (optimized) compilation has failed
+  // 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);
 };