Fix race condition with HasMutatorThread that was being used without a lock.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index 1bc179cdd3f696979ca6d252f09ef86522b1d08d..ca099b55956bdd8855f5fc6426ba32bef49144b8 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -1874,10 +1874,7 @@ void BackgroundCompiler::Run() {
         ASSERT(error.IsNull());
 #ifndef PRODUCT
         Isolate* isolate = thread->isolate();
-        // We cannot aggregate stats if isolate is shutting down.
-        if (isolate->HasMutatorThread()) {
-          isolate->aggregate_compiler_stats()->Add(*thread->compiler_stats());
-        }
+        isolate->aggregate_compiler_stats()->Add(*thread->compiler_stats());
         thread->compiler_stats()->Clear();
 #endif  // PRODUCT