Fix recent reload flakes

runtime/vm/isolate_reload.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/isolate_reload.h"
 
 #include "vm/become.h"
 #include "vm/bit_vector.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
@@ skipping 616 matching lines @@
 
     Dart_Handle retval = (I->library_tag_handler())(
         Dart_kScriptTag, Api::NewHandle(thread, packages_url.raw()),
         Api::NewHandle(thread, root_lib_url.raw()));
     result = Api::UnwrapHandle(retval);
   }
   //
   // WEIRD CONTROL FLOW ENDS.
   TIR_Print("---- EXITED TAG HANDLER\n");
 
+  // Re-enable the background compiler. Do this before propagating any errors.
+  BackgroundCompiler::Enable();
+
   if (result.IsUnwindError()) {
     if (thread->top_exit_frame_info() == 0) {
       // We can only propagate errors when there are Dart frames on the stack.
       // In this case there are no Dart frames on the stack and we set the
       // thread's sticky error. This error will be returned to the message
       // handler.
       thread->set_sticky_error(Error::Cast(result));
     } else {
       // If the tag handler returns with an UnwindError error, propagate it and
       // give up.
@@ skipping 70 matching lines @@
   if (!reload_finalized_) {
     Rollback();
   }
   CommonFinalizeTail();
 }
 
 
 void IsolateReloadContext::CommonFinalizeTail() {
   ReportOnJSON(js_);
   reload_finalized_ = true;
-  // As a final step, re-enable the background compiler.
-  BackgroundCompiler::Enable();
 }
 
 
 void IsolateReloadContext::ReportOnJSON(JSONStream* stream) {
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", "ReloadReport");
   jsobj.AddProperty("success", reload_skipped_ || !HasReasonsForCancelling());
   {
     JSONObject details(&jsobj, "details");
     if (reload_skipped_) {
@@ skipping 1155 matching lines @@
         ASSERT(!super_cls.IsNull());
         super_cls.AddDirectSubclass(cls);
       }
     }
   }
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart