Add a stress testing flag --background-compilation-stop-alot; add an extra check if background comp…

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 3250082caff8c11e5d85df941c4cbf4451be8c01..8061619223c93a768beb5e2c5bb7be19b293a6be 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -381,6 +381,8 @@ class Isolate : public BaseIsolate {
     return background_compiler_;
   }
   void set_background_compiler(BackgroundCompiler* value) {
+    // Do not overwrite a background compiler (memory leak).
+    ASSERT((value == NULL) || (background_compiler_ == NULL));
     background_compiler_ = value;
   }