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..549e211c05d4c78dd1dee98404ddcfd069efe5d1 100644
--- a/runtime/vm/compiler.h
+++ b/runtime/vm/compiler.h
@@ -94,6 +94,8 @@ class Compiler : public AllStatic {
   static RawError* CompileAllFunctions(const Class& cls);
 
   static bool always_optimize();
+
+  static void AbortBackgroundCompilation(intptr_t deopt_id);

[rmacnak, 2016/02/08 20:24:25]

Add comment that this is a transient failure for when the mutator changes state
we are using in a background compilation and we may retry the compilation later.

[srdjan, 2016/02/08 20:30:16]

  // 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.

 };