Address Siva's DBC

BUG=

Review-Url: https://codereview.chromium.org/2754113002 .
diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index a0542e5..b044daa 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -634,8 +634,6 @@
   // WEIRD CONTROL FLOW ENDS.
   TIR_Print("---- EXITED TAG HANDLER\n");
 
-  BackgroundCompiler::Enable();
-
   if (result.IsUnwindError()) {
     // We can only propagate errors when there are Dart frames on the stack.
     // TODO(johnmccutchan): Fix dartbug.com/29092.
@@ -725,6 +723,8 @@
 void IsolateReloadContext::CommonFinalizeTail() {
   ReportOnJSON(js_);
   reload_finalized_ = true;
+  // As a final step, re-enable the background compiler.
+  BackgroundCompiler::Enable();
 }